Implemented PQ for pseudo-pure to close https://github.com/CoolProp/CoolProp/issues/92

* As a consequence, made update function take CoolProp::input_pairs enum as first input (thanks @jowr for the idea)
* Decoupled the pre_update and post_update functions in order to allow for more creative update functions using guess values, etc.

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