Hybrid mixing coefficient

Merged Nathan Daelman requested to merge develop-hybrid-mixing into develop

As discussed in issue #851, I have connected the hybrid coefficient α (exact_exchange_mixing_factor) to the GUI. I re-used the XCFunctional.hybrid.parameter to store the coefficient in the run section, but standardized the key value. The updated parsers are VASP and FHI-aims with added tests.

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