Documentation of the space and new names for the cells

Some small improvements (hopefully !) to the code documentation and variable descriptions. Implements #208 (closed).

Added 10 commits:

• 820a53f1...d0033162 - 8 commits from branch master
• fe1e2304 - Merge branch 'master' into space_documentation
• 6ee3f611 - Fix couple of missing cells_top changes

Found an issue with MPI support and needed a merge to master. Otherwise looks good.

mentioned in commit ebb28e4d

Status changed to merged