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.
- merge branch with a method to control the speed of iteration and
improve the convergence of `warmdisp` (thanks Thomas)
- unify `diel_tens_fr` and `diel_tens_wr` into a single subroutine,
`dielectric_tensor`
- stay as close as possible to the notation of Daniela Farina's paper
- make `sox` an integer
- mark more subroutines as pure
- add more comments
