Unsymmetrical mixing functions

The casual user has no need to explicitly call this module.

.jfuncs provides different ways to evaluate the J function that appears in the unsymmetrical mixing terms of the Pitzer model.

One of these functions must be contained within the coefficient library in order to execute the Pitzer model functions:

• .P75_eq46 - P75 Eq. (46);
• .P75_eq47 - P75 Eq. (47);
• .Harvie - "Harvie's method", as described by P91, pages 124 to 125;
• .numint - using numerical integration method to evaluate J "exactly" (note: not currently autograd-able);
• .none - ignore the unsymmetrical mixing terms.