stall effects

[lacava]

track stalling in runs and act based on them.
stalling occurs when the engineered features are no longer improving either 1) the ML model CV score or 2) the median fitness of the features themselves.
if stalling occurs, there should be options to

• exit
• modify search to capture a different part of the search space. this could be achieved by increasing complexity of the features, increasing variation steps, or lowering selection presure.

[lacava]

fixed by #20