Add basic finite volume interface implementation

[greole]

This PR pulls the interface implementation of the geometric fields out of https://github.com/exasim-project/NeoFOAM/pull/55 and is part of https://github.com/exasim-project/NeoFOAM/pull/74

Besides adding the public API for some of the finiteVolume containers it implements some design decissions:

• The test folder structure should reflect the structure we have in include.
• Includes are composed to avoid one gigantic NeoFOAM.hpp. I would propose to have a 'core.hpp' next to core/ folder, which implements the #include core.... For now also cellCentred.hpp is included in NeoFOAM.hpp in the long term, this might be problematic if we also include vertexCentred.hpp which might have orthogonal implementations.
• It adds a namespace for finiteVolume
• It adds some constructors (to DomainField) for testing purposes. We might remove them later since they do construct DomainFields in an invalid state.
• The implemented tests are merely to see whether the includes are correct and don't test any functionality.

[HenningScheufler]

not added to the changelog