(Draft) Autocas interface as an IntegeralSolver

[alexfleury-sb]

Autocas implementation as an IntegralSolver. get_integrals still has to be coded (one-body integrals are easily retrieved from the results, but two-body integrals are still missing).

[alexfleury-sb]

This PR is left on the ice for some time to get other priorities done.

The current state:

• The autocas inputs are now handled by YAML. An interface with python dictionaries is existing, but only the basis set can be change for a given molecule (https://github.com/qcscine/autocas/blob/master/scine_autocas/__main__.py).
• It seems there is a bug for the basis set settings in YAML (it is ignored).
• We have to develop a way to get two-electron integrals.
• UHF and large CAS computation have not been tested yet.

[alexfleury-sb]

A minimal example has been merged into the Tangelo-Examples repo (sandbox-quantum/Tangelo-Examples#32).