SpaceGroup changes

[kaueltzen]

Summary

Fixes #3845 , #3861 and #3862 .

Includes the following changes in the symmetry module:

• The SpaceGroup symbol attribute now refers to its Hermann-Mauguin symbol with underscore notation of screw axes.
• The setting of the 7 rhombohedral space group types is now stored in an additional hexagonal bool attribute which can be set explicitly or by appending the setting to the int_symbol string parameter with the setting appended as before. The latter overrides the former for backwards compatibility.
• The SpaceGroup point_group attribute now always refers to the point group in Hermann Mauguin notation.
• A class method from_space_group was added to PointGroup which initializes the 32 crystal classes from a SpaceGroup string in Hermann-Mauguin notation.
• Changes of the space group databases symm_data.json and symm_ops.json were made which are documented in dev_scripts/update_spacegroup_data.py .
• If the full symbol is not available (for non-standard settings), a warning is now issued.
• Tests for the abovementioned points were added
• Notes on how to convert the old to the new symbol were added to docs/compatibility.md