changed regressions and weights tests to use API

Created by: wilsonmr

closes #459 (closed)

There are a couple of tests which I wanted to show can produce the same numbers in this commit but I think instead of creating a pd.DataFrame which we take values from to create a different one, the CSVs for relevant tests could just be changed to be the original CSVs (this would just be changing columns/indices) - These are marked with a TODO and should be done before merging

Most of the tests become simpler, there is the caviat that the fits test needs a work around which kind of defeats the point of the API but I felt like deleting as much as I could from conftest.py criticism welcome..