API - make BaseDatafit and BasePenalty regular classes

[Badr-MOUFAD]

Context of the PR

Currently, BaseDatafit and BasePenalty classes don't force their children to implement methods.
It is actually impossible to enforce that by deriving Base classes from abs.ABC since Numba Jitclass still doesn't support it.

Also, several datafits and penalties diverge from the design prescribed by Base classes as they implement other methods to comply with solvers.

This PR is meant to overcome this ambiguity.

Closes #202

Contributions of the PR

• Remove @abstractclass decorator from BaseDatafit and BasePenalty
• Remove unused methods in BaseDatafit and BasePenalty
• Update "add custom datafit/penalty" tutorials to reflect the changes

Checks before merging PR

• [ ] added documentation for any new feature
• [ ] added unittests
• edited the what's new

[mathurinm]

Thanks @Badr-MOUFAD , merge when green!