Investigate fold_tr_newp issue / Flexibility of transform_group

[mmcleod89]

In pregnancy module we need to take fold_tr_newp from hiv_status.

• This gets set in an internal self.fold_tr_newp in pregnancy as well in order to get around passing it into the transform_group method which only applies to dataframe columns
• Not sure if fold_tr_newp is const or not, but if it isn't then these have the potential to be out of sync --> better to define it in just one place
• We can get around this using bind/lambda but this may be a bit clunky
• We may want to alter transform_group (ideally in a backwards compatible way i.e. any additional variables having a default such as None
• We also want to rename fold_tr_newp to e.g. newp_transmission_factor or similar