XDMF methods for write and read (checkpointing)

This PR extends current XDMF interface with methods write_checkpoint and read_checkpointto write and read Function in parallel.

New functionality is based on existing HDF methods and data design is the same as in HDF5File class. Cell ordering, numbers of dofs per each cell, global dofmap and values for degrees of freedom are stored.

write_checkpoint prepares XDMF XML document, adds mesh via already existing add_mesh and adds function via newly created add_function. The add_function follows the pattern in HDF5File::write(Function).

read_checkpoint parses XDMF XML document, reads data items and calls HDF5Utility::get_local_vector_values. This method is common for both, XDMFFile and HDF5File and was separated out of HDF5File into HDF5Utility.

Write method allows appending already existing XDMF file with another function and/or adding several time-steps for function.

Unit test to check functionality for several finite elements are added with test to check time-series.

This PR appends new methods and should not break any existing functionality.