QM test updates.

Small fix to README

Removed an unneeded type

Small fix to xtb.go

Bugfixes

- Fixed a bug with a constant that caused int to overflow in Darwin (but not in Linux!)
- The fixing of atoms during geometry optimizations with xtb did not work. It does now.
- Small addition to the chemjson package (a function to produce a JSON-encodable error
  from a regular error.

Merge branch 'devel'

Merge commit that was only needed because I'm a mess