Collection of nitpicks and fixes

[marvinfriede]

This issue should serve as a collection of nitpicks and things to fix before it is released. Note that this should only include minor issue and no new features.

• [x] convenience function for total energy calculation (instead of singlepoint)
• [x] convenience wrapper for IndexHelper.from_numbers (abstract get_elem_param)
• [x] set dtype/device in tensors of new_<classical> directly
• [x] won't fix principal quantum numbers for Ba and Cs (see https://github.com/tblite/tblite/pull/123)
• [x] #131
• [x] decide on license
• [x] include license header in all files
• [x] remove data/PTB (only contains He at the moment)
• [x] remove data reader code
• [x] proper printout in CLI
• [x] fix SCF printout in batched mode
• [x] make double default dtype
• [x] improve SCF convergence thresholds
• [x] move coulomb electrostatics into interactions
• [x] move solvation into interactions
• [x] move dispersion into classical
• [x] write output to file or terminal
• [x] replace calculator options with config class
• [x] rename master branch to main
• [x] rename project from xtbML to dxtb (remove private old dxtb repo used for tbmalt)
• [x] proper __repr__s instead of __str__