Add further methods for calculating IC50.

[iansind]

These should be in the form of functions named after that specific equation.

[iansind]

@Ryan-Bringenberg - please use your subject matter expertise to determine if there are any additional ways of calculating IC50 or similar metrics that we should loop into the pyzyme umbrella.

[Ryan-Bringenberg]

@iansind - for very tight-binding inhibitors four parameter logistic fits are inadequate. We should consider adding Morrison tight binding. However, this is a rarely used equation and our time might be better spent elsewhere.

https://www.graphpad.com/guides/prism/latest/curve-fitting/reg_morrison.htm