refactor params

[jmercat]

It seems that the Params class is better in the params.py file than in model.py It allows importing params without importing the rest (which in some case might result in import loops...) Also Params contained some functions and class members instead of containing plain parameters. I simplified this to only containing parameters. It requires creating the function or class later at usage but it's not a big overhead.

This is a refactoring and might cause issues with dependencies...