Resolve differing references to I5a and I5b in income regressions

[andrewbaxter439]

Hi @dav-sonn extracting here as minor issue to look into -

from previous discussion:

a change across several places and it's hard to tell what should be 'I5b' and what should be 'I5a' (see other comment about loading from 'reg_RMSE.xlsx' also). Might need checked that the correct names are used in the correct places?

Originally posted by @andrewbaxter439 in #249 (comment)

[dav-sonn]

Thanks, Andy! I will do that as soon as I receive Daria's feedback.

[dav-sonn]

As instructed by Daria, I changed I5b_amount into I5a (consistently to the existing nomenclature of other processes). Now, the two distinct processes are named I5a and I5a_selection. Changes were pushed to the branch update/I5processes and merged it with develop (Pull request: #264).