add xtb driver to python drivers

[V-Alizade]

• implement xtb driver with tblite library
• add an example for the GFN2-xTB based on methane

[mahrossi]

This looks quite tidy to me! Thank you for the contribution. I would like to find a bit of time to try out the example in practice. Will set the automatic checks to run now.

[mahrossi]

This msg is not consistent with the pip installation that is suggested in the README (pip installation). What is best?

[V-Alizade]

It seems tblite has better support with mamba for Mac than with pip. I changed the README to mamba.

[mahrossi]

Hi I could not install tblite on my mac computer, so I failed to test it. The meson build has linking issues. I did not have time to dig it out. So I could not test explicitly.

[mahrossi]

I forgot about this - sorry - I think it can be merged.

[ceriottm]

@V-Alizade just a heads-up - we are revamping the PES infrastructure, so that it is also possible to run straight from i-PI without a driver (which is useful when running on a single node, and e.g. plain MD).
in doing so I had to change the way the xtb driver works, by reading the config from a json file rather than a json formatted string on the command line. Comment on the #390 PR if you have issues on the tblite front with what I did.

[V-Alizade]

@ceriottm I checked xtb client after your update, and everything looked fine. I also created an update to make it easier to use, please see #407