Improvement: Better handling of hashes and errors in t8_geometry [2/2]

[holke]

Closes #1657
Requires merging of #1662 first.

Describe your changes here:

I stumbled on this while testing something else and noticed that if a tree has no geometry, but we query for it (for example when trying to print VTK), we run into a segfault.
While working on this i noticed that the handling of hash values of geometries should be improved.

• Added new header that defines hash handling functions.
• New strong data type for hashes (a strong type version of size_t)
• Definition of empty hash, that is a hash for the string "" which we do not allow as name of a geometry and use as hash for a non-existing geometry
• definition of INVALID geometry
• proper error handling when no geometry is registered but asked for. In that case the functions return nullptr
• proper handling of cmesh vtk output not reading vertex coordinates
  • Error message and not writing the file for cmesh ASCII output
  • Error message and abort for writing cmesh VTK API output
• Added a test with a cmesh with no geometry to check that nullptr is returned.

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