Add cell diameter

[Jan Blechta]

Add CellDiameter node to UFL and implement it as equal to MaxCellEdgeLength for simplices and max(distance(v0, v1) for v0 in vertices(cell) for v1 in vertices(cell)) for non-simplices.

[Jan Blechta]

Opinions @martinal, @miklos1?

[Miklós Homolya]

The proposed max(distance(v0, v1) for v0 in vertices(cell) for v1 in vertices(cell)) formula is not quite correct for cells with bendy boundaries, and it is not quite clear what the correct calculation is.

[Miklós Homolya]

Firedrake users generally settle with CellVolume(mesh)**(1/dim) as an approximation for CellSize(mesh), although this sadly does not take into account aspect ratio of the cell.

[Jan Blechta]

I totally agree, @miklos1. We can raise a warning or an error for bendy cells. That's for discussion.

[Jan Blechta]

Resolved by pull request #89. Merged at 9f6dbf5.