Hashes to identify precomputed properties on a mesh

[hermannsblum]

As originally discussed here:

To cache the computation of the map planes, a nice abstraction would be that you just always call this module in the same way and then check whether you have a stored result for the given mesh file or otherwise produce and store it. For a start, you can probably just identify a mesh by the number of vertices or so. An improvement would then be to take a hash of the mesh to have a good statistic whether something changed.

Originally posted by @hermannsblum in #86

By computing hashes over meshes, we can re-identify them and therefore load corresponding precomputations. As an approximation, we are using the number of vertices for now in the global localisation, but this is not really valid and should be changed before release.