add support to pair_modify to selectively disable compute/tally callbacks in sub-styles for pair hybrid and hybrid/overlay

- put a note into the manual
- reorder contents of the README file
- request for information should be sent to lammps-users
- add list of known missing features

Prevent segfault if Python was never initialized

User manifold fix every keyword

Extract for bond

Minor updates to docu of USER-CGDNA package

collected small updates and bugfixes

Support mixed Python use by honoring Python GIL