Add the ability to set component-specific, field-based diffusivities.

[ecoon]

There is a feature request to support hacks at gas diffusion. The idea is to make a diffusivity that is:

1. component specific (e.g. all other components have their usual scalar, fixed value diffusivity)
2. field and evaluator-based, allowing generic functions of other vectors (e.g. saturation)

[ecoon]

Working on this on branch ecoon/neel_gas_diff