There are two cases in which a wave in GRAY can reach and pass through a
cutoff:
1. a discontinuity or too large step size causes the integrator to jump
into region with N⊥² < 0, according to the cold plasma theory.
2. a wave gets close to a cutoff region is still propagating according
to the cold plasma theory, but the warm disperion relation already
gives re(N⊥²) < 0.
For case 1. we raise an error and immediately stop the raytracing as it
must be completely invalid. For case 2. we force N⊥² = 0 to avoid
interpreting the wave reflection as a strong energy absorption, but
allow the raytracing to continue.
- rename errocodes → gray_errors
- restructure the errors into a `error_spec` type
- make the list of errors easily extensible
- rewrite the `print_errn`, `print_errhcd` (now `print_err_raytracing`,
`print_err_ecrh_cd`) subroutine to handle arbitrary errors
- add functions to easily manipulate errors
(`raise_error`, `has_error`, `is_critical`)
- remove print statements from quadpack
- log all errors to stderr using the logger module
- 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
