Support more file formats for input/output

[awvwgk]

Allows xtb to read new file formats. Solves #95 on the xtb level.

• read and write Gaussian external input files as documented here
  • find out default file ending to activate automatic detection (@pierre-24 can I get some input?)
• write Turbomole lattgrad file
• write energy/gradient to Orca engrad format
• write energy/gradient to Gaussian external format

[pierre-24]

Here is an example of *.Ein and *.Eou files for the NH3 molecule: https://gist.github.com/pierre-24/21ebf089b3f2ffa77dd6789175a8a542

[pierre-24]

Note that you will still requires a (lighter, tough) interface, since as you can see in the "script invocation tab", Gaussian just pass a bunch of different files as input to the program, but thanks anyway ;)

[awvwgk]

A small wrapper script like below should suffice:

#!/usr/bin/env sh
exec xtb $2 --gaussian $3 &> $4

[pierre-24]

That is also more or less what I had in mind :)

[awvwgk]

Huh? A GH error, this is something unusual.

[awvwgk]

@pierre-24 Would you mind testing the Gaussian interface? I don't have access to this program.