updated Plots doc

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Added Tsat property back to Python State class

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Beginning work on 2phase splines for better interpolation

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Merge branch 'master' of https://github.com/ibell/coolprop
Merge branch 'master' into TTSE_2phase_spline
Added spline code for 2phase in TTSE (few orders of magnitude improvement in error over old method)

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Extended some 2phase properties for TTSE all the way to the critical point.  Still need to calculate dsdp, dhdp, drhodp along sat. at the critical point.  That should solve the problems in critical region.

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Merge branch 'master' of https://github.com/ibell/coolprop