Build temporary facet sets for constructing facet-dof maps

We can't define the dats on sets with halos because when we halo
exchange we get the remote process's numbering, rather than our own.
Instead just build temporary sets without halos that are total_size
big.
Cosmetic change: avoid one variable having two values in the same method

The variable facetset pointed to two different sets in the methods _build_interiorfacet2dofmap_facets() and _build_interiorfacet2dofmap_coords().
Fix calculation of global number of cells

The global number of cells and hence the parameter alpha was not calculated correctly and this led to inconsistencies between serial and parallel runs. To fix this, calculate the number of cells via allreduce'ing the local cell sizes.
Merge branch 'lumpedmass-parallel'