Warn about missing IndexMap in Topology

[chrisrichardson]

Throw a more appropriate error when there is a missing IndexMap in Topology.

Resolves #3844

[schnellerhase]

Should we throw in index_maps right above instead, if maps is empty? Then we get the same interface behaviour for mixed topologies.

[chrisrichardson]

I don't think that would be right. It might be that you want to check if there are any IndexMaps of dim(1) or not, without throwing an error.

[schnellerhase]

Then we would have different behaviours between index_map and index_maps, it would be clearer if the behaviour was equivalent up to signature and return type. Even more on the Python side both functions will raise if the entities have not been computed before - #3756 (comment). So, with the new changes index_map would both in Python and C++ raise and index_maps only on the Python side but not in C++.

A quick check also does not show that we functionally rely on being able to check for entities having been computed beforehand main...schnellerhase/test-raise-indexmaps.