convey PL ~ DP regression model

[mlin]

To help with approximate reconstruction of genotype likelihoods from binned DP (when downstream tools strictly require them to be present even in non-variant cells):

1. during encoding, calculate regression coefficients for PL[1] and PL[2] on binned DP
2. include them in the output file, perhaps in the header (but that'd require a second pass)
3. optionally use them to fill out approximate PL vectors during decoding