[Users] GRHydro: explicit evolution of the electron fraction

[Christian D. Ott]

Hi,

in the process of making GRHydro work out of the box
with microphysical EOS, I would like to introduce the electron fraction
Y_e (a HydroBase variable) as a variable that is explicitly evolved via
a conserved variable consY_e = dens * Y_e by GRHydro (by setting
Y_e_evolution_method = "GRHydro").

Previously, Y_e was evolved as the first of the advected scalar tracers.
This is currently broken, since Y_e in HydroBase and the first tracer
are not mapped to on another. The tracer variables will (in the future)
be needed to consistently evolve mass fractions of various elements (for
use in a low-density EOS and in a nuclear reaction network) that must
sum up to one without Y_e. Moreover, in nuclear statistical equilibrium,
Y_e is the only compositional variable needed.

Hence, it makes sense to promote Y_e to special status.

Please speak up if you have objections.

 - Christian