Try to isolate the core of pyphenology

[Peter9192]

Work in progress. Peeling off the outer shells of pyphenology to expose the core functionality.

Pyphenology does a lot of data handling and validation before passing data into the core routines that fit the functions. However, the separation between the core and the outer shell is a bit lacking. A lot of state is being set in the different layers of the source code, which makes it hard to understand what's going on, and to try to use a different layer (more sklearn like) to directly interact with the core