remove the init() method from the HGF base class

[LegrandNico]

We add ad hoc create and cache methods instead that are called by default when using input_data, but are still available to the user if this step should be performed before model fitting.

This will close #142.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.34%. Comparing base (84e6def) to head (b395f77).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #171 +/- ## ========================================== + Coverage 93.24% 93.34% +0.09% ========================================== Files 17 17 Lines 859 871 +12 ========================================== + Hits 801 813 +12 Misses 58 58 ```

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