Add geometric fields documentation

[greole]

This PR pull @HenningScheufler documentation from https://github.com/exasim-project/NeoFOAM/pull/55 branch to a separate branch for easier review.

Needs:

• or https://github.com/exasim-project/NeoFOAM/pull/72

[bevanwsjones]

I think it would be a bit helpful if we had some basic class diagrams, perhaps we should also try to give the different fields a more mathematical description. I think this will give more clarity, it also normally highlights inconsistent thinking and logic in class design. Not all this is needed now, but just as a general comment.

e.g.

• domain field represents some tensor field in a Euclidean domain omega,
• boundary fields represent a tensor field on the boundary of the Euclidean domain and is denoted partial omega (and is thus subset of omega).
• The internal field is the field of tensors which is the set difference of omega and partial omega.

I find set notation and theory are very good at creating clear definitions for meshing/spacial concepts. This makes it easy to know where to put things and what to call them.