This is a tracking issue to integrate the tblite library with xtb.

To get the PBC support in xtb finally in a usable state we have to integrate the tblite library which started as a spin off to support the xTB methods in DFTB+.

This will enable:

• customizable and programmatically accessible parameter files (breaking change in the parameter format, again)
• periodic boundary condition support for GFN1-xTB and GFN2-xTB

Things to take care of first:

• integration of solvation models, here in xtb or in library?
  Support solvation models tblite/tblite#23
• external potentials, defined by xtb or in library?
  Support external point charges tblite/tblite#22
• support for GFN0-xTB in library?
  Support for non-selfconsistent xTB methods tblite/tblite#29