Allow automatic choice of split factor

[hanno-becker]

Context: When optimizing large code using the split heuristic, a rough rule of thumb is to pick the split factor so that each window has a size somewhere between 50-200 instructions. It is curently up to the user to make this calculation and configure SLOTHY appropriately.

Task: Allow the configuration of a 'split heuristic threshold', which is a codesize above which split heuristic should be applied automatically, picking the split factor so that the window size is as specified by the threshold. For example, is split_heuristic_threshold=150 and one is optimizing code of length 400, it should automatically pick split_factor=2.66.

[hanno-becker]

cc @aqjune