This change adds a bit of documentation and simplifies the two
(internal) subroutines used to find the horizontal tangent points
and the magnetic O/X point.
Using a closure we can avoid explicitly passing parameters (psi0) to
hybrj1. Previously this required a custom `hybrj1mv` subroutine in
fitpack with an identical interface, except for our extra parameter.
This change replaces the output files (Fortran units) with a derived
type called table, that hold the data in memory until further
processing. The data stored in a table can be dumped to a file, as
before, or processed in other ways, for example converted to other
derived type.
