Parsing hybrid functional parameters

General explanation

Hybrid functionals have, by definition, a mixing parameter (typically abbreviated as alpha) that determines the fraction of Hartree-Fock/Exact Exchange mixed in. All libxc names come with defaults (most often alpha = 0.25), but many codes also allow the user to tweak these at will. Actually, it is quite common to recalibrate the alpha to a specific use case/observable. Hence, the parsers should be able to at least capture this value.

Similarly, the special subclass of short-range hybrid functionals introduce a screening parameter (abbreviated as omega) that cuts off the Exact Exchange after a certain range. This parameter too is common enough that it has to be captured.

Specific requests

1. Parse and store the alpha parameter as a fraction (i.e. ]0,1[) in all electronic structure code that supports hybrid functionals, when present.
2. Likewise, parse and store the omega parameter in Ångstrom.
3. Default values do not have to be exposed (yet).

xc_hybrid_keywords contains the code-specific keywords (to be updated in the future).