Use mctc library for geometry input / output

[awvwgk]

Another step to an uniform geometry handling library for our computational chemistry projects. This patch removes the geometry readers from xtb and replaces them with the implementations available in mctc-lib.

This gives xtb automatic support for

• QCSchema JSON files
• SDF files with V3000 formatted connection tables
• FHI-aims geometry.in files
• Q-Chem molecule files
• future implemented formats in mctc-lib

Todo:

• synchronize the vasp_info, turbo_info and struc_info types (xtb) with the structure_info type (mctc-lib)
• catch structures not supported by xtb:
  • 1D/2D periodic structure (tmol, aims)
  • helical objective boundary conditions (gen)
  • 2D flat molecules (ctfile)
  • hydrogen atom queries (ctfile)
  • protein missing hydrogen (pdb)
• replace writer routines with mctc-lib provided ones
• check geometry conversion factors in mctc-lib against xtb (different CODATA versions)
  Include CODATA2018 constants mctc-lib#37