Add a data loss function for LogBinner (and ErrorPropagator)

[ffreyer]

A given binning lvl consists of data chunks of size 2^(lvl-1), which means that out of N values N % 2^(lvl-1) will be missing. This should be equivalent to logbinner.count[1] - logbinner.count[lvl]. It would be good to have a function returning this amount.