Feat/exp initial difficulty [WIP]

[L-M-Sherlock]

Weighted average by reviews:

Algorithm	Log Loss	RMSE (bins)	Parameters
FSRS-4.5	0.3252±0.1514	0.0533±0.0334	17
FSRS-4.5 + exp init d	0.3250±0.1515	0.0530±0.0333	17

Weighted average by log(reviews):

Algorithm	Log Loss	RMSE (bins)	Parameters
FSRS-4.5	0.3485±0.1701	0.0733±0.0474	17
FSRS-4.5 + exp init d	0.3483±0.1702	0.0729±0.0474	17

improved ~0.6% and ~0.5%.