Simplify naming convention conversion between pydeseq2 and formulaic

[jeandut]

This information is already available from formulaic directly. When design is your design matrix, there is

design.model_spec.variable_terms

and

design.model_spec.term_variables

to map from variables to formulae terms and vice versa.

Originally posted by @grst in #181 (comment)

The complex logic used to handle interaction terms could indeed be simplified (as long as this logic is still needed).