Add a global option for binding models to select titrant mixing model

[jchodera]

Would be useful to have a categorical selection of titrant mixing model, e.g.:

• perfusion with instantaneous mixing
• perfusion with post-injection mixing
• non-perfusion

These could be separated out into separate functions that take mixing argument so that you don't need to keep reimplementing.

Later, we can do automatic selection of the model that best fits data.