Added reduceAndReindex-function to separate out the reindex stage.
Added test function to compute the number of faces in a cell.
Extraction and building of faces in subgrids are working.
Comments and formatting fixes.
Fixed bug in the face indexing of SubGridBuilder::extractNeighborFaces.

* Properly invert the set.
* Added test that triggered the bug.
Use unordered_map in SubGridBuilder::extractNeighborFaces.
Added test that check the proper reindexing of normals.
Added SubGridBuilder::extractNeighborNodes.
Properly copy UnstructuredGrid::node_coordinates when making SubGrid.
Merge branch 'master' of https://github.com/sintefmath/equelle