weights utilities + fix

[Aske-Rosted]

Adds 2 options for the weight fitter

• a choice to employ an automatic log10 binning
• a choice to cap the max single event weight to be x * sum(all_weights) (1>x>0)

Adds an option for the BjoernLow class

• determine x_low as a percentile of the truth. x_low in this case has to be given as a value between 0 and 1.

• a fix so that the db_count_norm works as intended.

[RasmusOrsoe]

@Aske-Rosted - I think the changes look good. Two of the doc string lines are too long though ( see code climate). Could you shorten those? The other codeclimate warnings are fine.