Updated critical spline

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Implemented passing C++ unit test for T->p->T round trip consistency

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Added new test running function and refactored other ones

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
fixed typo in REFPROP test

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
More internal debugging of Brent's method

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Updated docs for cython Props and PropsSI

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>