add variance inflation factor calculation to Data
move base models classes from old code to new version
update code in order to improve pep8
Merge branch 'develop' of github.com:pavlov99/pmll into develop
update docstrings
remove extra files
move regression from previous code to current
update README, order fields
add least squares method to library
return object if slice one element rather than Data instance
fix one element indexing problem
simplify code
make feature callable
add hash to features, so it is possible to use set of features. Add __add__ functionality to Data, it concatenates features
temporary remove python3 from supported versions