This change combines `equian` and `equinum_psi` into a new `pol_flux`
subroutine that computes ψ(R,z) and derivatives for either numerical or
analytical equilibria.
Similarly, `equian` (the fpol and dfpol outputs) and `equinum_fpol` are
combined intro `pol_curr` that computes F(ψ) and its derivative.
Callers of these subroutines do not select a specific version based on
the value of `iequil` anymore.
In the case of analytic equilibrium without a limiter contour, the
simple limiter was built incorrectly due to an unnecessary conversion
from cm (the equilibrium data are already in metre).
This adds a new `splines` module which implements a high-level interface
for creating and evaluating splines and rewrite almost all modules to
use it. Also, notably:
1. both `simplespline` and DIERCKX splines can now used with a uniform
interface
2. most complexity due to handling working space arrays is gone
3. memory management has been significantly simplified too
1. Fix the mismatch between the psnbnd in coreprofiles and gray_core.
This happens whenever gray overrides the externally provided one
(i.e. the density tail would become negative before psnbnd and is so
rescaled to end exactly on the zero).
2. Make psnbnd no longer required by always computing it as in 1.
It hasn't been removed, because gray_params.data is sacrosant,
but it no longer has any effect.
3. Cleanup: mark public functions, restructure the global variables into
three categories; add comments explaining the analytical profiles
format, formulae and how the polynomial tail is computed.
