Made NoMixing a class.

For uniformity with its sibling classes.
Made phase mixing functors a class hierarchy.

In summary:
 - added RsFunction (base class),
 - made NoMixing, RsVD, RsSatAtContact inherit RsFunction,
 - RS and RV are no longer template arguments for EquilReg class,
 - EquilReg constructor now takes two shared_ptr<Miscibility::RsFunction>,
 - use of constructor updated, mostly using make_shared.
Removed redundant calcPressII() method.

Pressure is also calculated in the calcPressSat() method.