Reads ffprime, pprime from geqdsk file;
passes through to regions, calculates Jpar0
and saves to the output grid

H.Seto noted that in the COCOS +1 convention that Hypnotoad assumes,
Jpar has a minus sign.

Used to detect when outside the core plasma. Test cases have no
X-points so no psi_bdry.

Use Scipy.interpolate to differentiate w.r.t psi and save
to the output geqdsk file.

Jpar0 isn't in the reference grid files so integrated tests fail.

InterpolatedUnivariateSpline requires monotonically increasing X axis,
so reverse sign of psi in negative Bp case.