Automate calibration of additive factors in puf_data/impute_itmexp.py

[martinholmer]

This discussion in the review of pull request #275 and some recent investigation of the scipy package, suggests the possibility of a major enhancement to the puf_data/impute_itmexp.py logic. Instead of hand calibration of the additive factor af variable values, it seems as if the root finding can be automated using scipy.optimize capabilities. This enhancement would have several benefits: the logic of puf_data/impute_itmexp.py would be much clearer than it is now. And, once automated, the developer time required to calibrate the af variables would be greatly reduced. But the biggest benefit of this enhancement would be the developer skill learning effect. In order to get this final benefit, someone else need to develop the pull request that automates the af calibration. I'll be happy to be an advisor and answer questions that come up during the development work.

@MattHJensen @andersonfrailey @hdoupe @Amy-Xu