Process pipeline richard

[richardyz98]

Hey Matt and Sebastian, This is the latest PR with fixed included. These include:

• all clerical fixes regarding variable names etc.
• Fixes to the maxcut tests
• Testing and implementation of priors in the average missing data imputation
• Changing perform_split to take strings and respecting the string sample interface
• Changing transform_priors to take a string indicating the transformation, and implementing a set of functions