XDMF writer: share mesh between different functions (avoid ParaView oddities)

Thanks to PR #338, FEniCS has support for Xdmf3 time series with multiple data sets.

So far, the data sets are stores as

<?xml version="1.0"?>
<!DOCTYPE Xdmf SYSTEM "Xdmf.dtd" []>
<Xdmf Version="3.0" xmlns:xi="http://www.w3.org/2001/XInclude">
  <Domain>
    <Grid Name="TimeSeries_u" GridType="Collection" CollectionType="Temporal">
      <Grid Name="mesh" GridType="Uniform">
        <Topology NumberOfElements="2121" TopologyType="Triangle" NodesPerElement="3">
          <DataItem Dimensions="2121 3" NumberType="UInt" Format="HDF">all.h5:/Mesh/0/mesh/topology</DataItem>
        </Topology>
        <Geometry GeometryType="XY">
          <DataItem Dimensions="1149 2" Format="HDF">all.h5:/Mesh/0/mesh/geometry</DataItem>
        </Geometry>
        <Time Value="0" />
        <Attribute Name="u" AttributeType="Vector" Center="Node">
          <DataItem Dimensions="1149 3" Format="HDF">all.h5:/VisualisationVector/0</DataItem>
        </Attribute>
      </Grid>
      <Grid>
        <xi:include xpointer="xpointer(//Grid[@Name=&quot;TimeSeries_u&quot;]/Grid[1]/*[self::Topology or self::Geometry])" />
        <Time Value="0.001" />
        <Attribute Name="u" AttributeType="Vector" Center="Node">
          <DataItem Dimensions="1149 3" Format="HDF">all.h5:/VisualisationVector/3</DataItem>
        </Attribute>
      </Grid>
     <!-- More time steps ... -->
    <Grid Name="TimeSeries_p" GridType="Collection" CollectionType="Temporal">
      <Grid Name="mesh" GridType="Uniform">
        <Topology NumberOfElements="2121" TopologyType="Triangle" NodesPerElement="3">
          <DataItem Dimensions="2121 3" NumberType="UInt" Format="HDF">all.h5:/Mesh/1/mesh/topology</DataItem>
        </Topology>
        <Geometry GeometryType="XY">
          <DataItem Dimensions="1149 2" Format="HDF">all.h5:/Mesh/1/mesh/geometry</DataItem>
        </Geometry>
        <Time Value="0" />
        <Attribute Name="p" AttributeType="Scalar" Center="Node">
          <DataItem Dimensions="1149 1" Format="HDF">all.h5:/VisualisationVector/1</DataItem>
        </Attribute>
      </Grid>
      <Grid>
        <xi:include xpointer="xpointer(//Grid[@Name=&quot;TimeSeries_p&quot;]/Grid[1]/*[self::Topology or self::Geometry])" />
        <Time Value="0.001" />
        <Attribute Name="p" AttributeType="Scalar" Center="Node">
          <DataItem Dimensions="1149 1" Format="HDF">all.h5:/VisualisationVector/4</DataItem>
        </Attribute>
      </Grid>
     <!-- ... -->
    </Grid>
  </Domain>
</Xdmf>

Every data set (here u and p has its own TimeSeries.

This has multiple disadvantages:

The mesh cannot be shared between u and p.
ParaView recognized the data sets as partial, leading to all sorts of funny errors (see, e.g., here).

In case the the data sets share the same mesh – which I assume is the dominant use case –, the data could be stored like

<?xml version="1.0"?>
<!DOCTYPE Xdmf SYSTEM "Xdmf.dtd" []>
<Xdmf Version="3.0" xmlns:xi="http://www.w3.org/2001/XInclude">
  <Domain>
    <Grid Name="TimeSeries_u" GridType="Collection" CollectionType="Temporal">
      <Grid Name="mesh" GridType="Uniform">
        <Topology NumberOfElements="2121" TopologyType="Triangle" NodesPerElement="3">
          <DataItem Dimensions="2121 3" NumberType="UInt" Format="HDF">all.h5:/Mesh/0/mesh/topology</DataItem>
        </Topology>
        <Geometry GeometryType="XY">
          <DataItem Dimensions="1149 2" Format="HDF">all.h5:/Mesh/0/mesh/geometry</DataItem>
        </Geometry>
        <Time Value="0" />
        <Attribute Name="u" AttributeType="Vector" Center="Node">
          <DataItem Dimensions="1149 3" Format="HDF">all.h5:/VisualisationVector/0</DataItem>
        </Attribute>
    <Attribute Name="p" AttributeType="Scalar" Center="Node">
          <DataItem Dimensions="1149 1" Format="HDF">all.h5:/VisualisationVector/1</DataItem>
        </Attribute>
      </Grid>
      <Grid>
        <xi:include xpointer="xpointer(//Grid[@Name=&quot;TimeSeries_u&quot;]/Grid[1]/*[self::Topology or self::Geometry])" />
        <Time Value="0.001" />
        <Attribute Name="u" AttributeType="Vector" Center="Node">
          <DataItem Dimensions="1149 3" Format="HDF">all.h5:/VisualisationVector/2</DataItem>
        </Attribute>
    <Attribute Name="p" AttributeType="Scalar" Center="Node">
          <DataItem Dimensions="1149 1" Format="HDF">all.h5:/VisualisationVector/3</DataItem>
        </Attribute>
      </Grid>
     <!-- More time steps ... -->
  </Domain>
</Xdmf>

Advantages:

(slightly) smaller files
better support for ParaView.

For implementing this, I see two possible interfaces:

Provide a parameter (like the exisitng 'rewrite_function_mesh') that tells the writer to share the mesh between all input functions
Add a write_all method with the signature write_all(list_of_functions, time).

The latter is perhaps cleaner in that it forces all functions to be present at all time steps.

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