ABC the splitting classes

[karel-w]

Summary

Create an abstract base class for all the splitting classes, such that we can recycle functionality required for splitting.

Maybe we still want to add more splits to see if more patterns emerge.

Motivation

In our splits.py:

TODO
----
- Introduce parent Splitter class when splitter patterns emerge.

One of the patterns emerging now is the validation of sequence overlap, i've added this to the PredefinedSplitter.

Proposed solution

ABC for the splitter, e.g.,

class AbstractSplitter(ABC)
        def _validate_no_sequence_overlap()

class RandomSplit(AbstractSplitter)
class KfoldSplit(AbstractSplitter)
class PredefinedSplitter(AbstractSplitter)