Xtb

Maintenance and bugfix release. Thanks to Christoph Bannwarth (cbannwarth) and Jan Unsleber (nabbelbabbel) for contributing to this release.

- Bugfix: JSON dump did not return correct version number :beetle:
- Bugfix: Sign error in printout for GFN1-xTB Mulliken charges :beetle:
- Bugfix: PDB charges were not written correctly in PDB output :beetle:
- Bugfix: SRB strain derivatives were wrong :beetle:
- Bugfix: EEQ returns correct energy for one atom systems :beetle:
- The error handling has been completely reworked
- Improved implementation of CM5 charges
- Analytical gradients for the Halogen-bond correction
- Benzene is now available as synonym for toluene for GFN2/GBSA
- Massively improved C-API
- Gaussian external input and output formats
- CMake support for building `xtb`
- GCC support for building `xtb` with both meson and CMake

This release is API compatible to version 6.2.2

Addendum: `otool_xtb` bash script to fix incompatibility of this release with the Orca 4.2.1 interface

v6.2.2-hotfix
Maintenance, bugfix and hotfix release.

- configuration scripts are back in xtb
- pkg-config file is now available
- DFTB+ genFormat is supported as geometry input
- additional check to prevent calculations on invalid geometry input
- RF-Optimizer will not read hessian file by default anymore (now really)
- GFN0-xTB can now be correctly requested from the $gfn group
- Thermodynamic functions will not attempted to be calculated for T=0K anymore
- Bugfix: periodic GFN0-xTB used wrong i-i' D4 dispersion interactions

This release is API compatible to version 6.2.1

Maintenance and bugfix (:beetle:) release.

- add version number notice including git commit
- optional arguments for the C-API using C `NULL`/C++ `nullptr`
- allow multiplicity in API
- add GBSA solvation for GFN0 via API
- allow GFN1 interface to return partial charges, dipole moments, bond orders, ...
- custom solvent parameters and preloading from API
- GBSA solvation free energy, Born-radii and SASA are available in Python
- Bugfix: GBSA `raise` calls used wrong interface :beetle:
- Bugfix: C-API calls to calculators used wrong interface :beetle:
- Bugfix: SDF input read in the wrong order :beetle:
- Bugfix: thermodynamic functions including isotope information :beetle:

This release is breaking API compatibility to version 6.2

First GH release of `xtb`, so we are finally open source now. For older version, compare the changelog at [read-the-docs](https://xtb-docs.readthedocs.io/en/latest/versions.html)

- Bugfix: Fukui index calculation :beetle:
- Bugfix: wrong forces in FIRE optimizer :beetle:
- Bugfix: $cube instructions were not read :beetle:
- Bugfix: Input error for $gbsa data group :beetle:
- GFN0-xTB Hamiltonian consistent with [ChemRxiv preprint](https://dx.doi.org/10.26434/chemrxiv.8326202.v1)
- periodic boundary conditions for GFN0-xTB
- preliminary implicit solvation model GBSA for GFN0-xTB