Add range over node indices

what do you think about the naming of these functions. there is some discussion in dune-grid about ranges over subentities. We could leave it as in this MR or change to the convention

• nodeIndex: a single index
• nodeIndices: the number of indices
• nodeIndexRange: for the range over node indices

Let's change it to your suggestion there. At least it is somewhat consistent with the current dune naming scheme.

added 1 commit

• 6f44dd6d - add nodeIndexCount and test of nodeIndex range

Compare with previous version

I have decided to go another route more compatible with the current status-quo of dune-grid:

• nodeIndices: a range of the node indices
• nodeIndexCount the number of indices

enabled an automatic merge when the pipeline for 6f44dd6d succeeds

merged

mentioned in commit 4e38a30a