Small improvements in multimesh sparsity pattern builder.

Runs very slow but slowness seems to be due to these lines
in SparsityPattern.cpp:

for (i_index = map_i->begin(); i_index != map_i->end(); ++i_index)
      diagonal[*i_index].insert(map_j->begin(), map_j->end());

where both map_i and map_j are large (size = 64).
Merge branch 'oyvinev/fix-tabulate-all-coordinates'
Improve documentation of JIT compiled Expression wrt passing a mpi_comm
Merge branch 'johanhake/mpi-group-support-in-jit_compilation'
Keep backward compatibility for reading functions from HDF5 files.
Merge branch 'oyvinev/hdf5-keep-backward-compatibility' into next