smolar added to python output of phase envelope

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Updated docs and cross-linking between them

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Added Q to SimpleState for use in phase envelope

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Both xN independent and dependent are now tested in mixture derivative code.

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Added first cut at PT and P&HSU solvers - much work to be done, currently only good in gas phase.

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