Similarly to eb648039 this change replaces the `equilibrium` module with
a new `gray_equil` module providing the same functionality without using
global variables.
- `read_eqdsk`, `read_equil_an` are replaced by a single `load_equil`
routine that handles all equilibrium kind (analytical, numerical,
and vacuum).
- `scale_equil` is merged into `load_equil`, which besides reading
the equilibrium from file peforms the rescaling and interpolation based
on the `gray_parameters` settings and the equilibrium kind.
To operate on G-EQDSK data specifically, the `change_cocors` and
`scale_eqdsk` are still available. The numeric equilibrium must then
be initialised manually by calling equil%init().
- `set_equil_spline`, `set_equil_an`, `unset_equil_spline`
are completely removed as the module no longer has any internal state.
- `fq` is replaced by `equil%safety`; `bfield` by `equil%b_field`;
`frhotor`, `frhopol` by `equil%pol2tor` and `equil%pol2tor`;
and the remaining subroutines by other methods of `abstract_equil`
retaining the old name.
- the `contours_psi` subroutine is replaced by `equil%flux_contour`,
with a slightly changed invocation but same functionality.
- the `gray_data` type is no longer required ans has been removed: all
the core subroutines now access the input data only though either
`abstract_equil`, `abstract_plasma` or the `limiter` contour.
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