This change replaces the `coreprofiles` module with a new `gray_plasma`
module providing the same functionality without using global variables.
- `read_profiles`, `read_profiles_an` are replaced by a single `load_plasma`
routines that handles both profiles kind (numerical, analytical).
- `scale_profiles` is merged into `load_plasma`, which besides reading
the profiles from file peforms the rescaling and interpolation based
on the `gray_parameters` settings.
- `set_profiles_spline`, `set_profiles_an`, `unset_profiles_spline`
are completely removed as the module no longer has any internal state.
- `density`, `ftemp`, `fzeff` are replaced by the `abstract_plasma`
type which provides the `dens`, `temp` and `zeff` methods for
either `numeric_plasma` or `analytic_plasma` subtypes.
