Add Entity/EntityRep ID comparison tests

[SoilRos]

This patch adds a test that compares the ids and indices for Entity and EntityRep. The idea is that we can split the two functionalities (because the are different) without breaking the current functionality.

This is a re-work of the test introduced by @aritorto in https://github.com/OPM/opm-grid/pull/905 and https://github.com/OPM/opm-grid/pull/898, but without the modifications to the index set yet.

From what I understand, the desired behavior is that Entity and EntityRep have the same global/local id at the level zero when the grid is not distributed. Otherwise, the ids should be different. This test shows that that is the case for local ids, but fails for global ids when the grid is in parallel.

The test case is marked with TODOs on the places that are not currently working according to my testing.