reports dryclean in read count units and input signal retained in outputs

[jrafailov]

Description:

• foreground was previously reported in linear space, mean/median-normalized to 1
• foreground is now reported in read count units.... or in the correct units relative to input field (fragcounter reads)

Changes:

• migrated centering of sample to prep_cov
• input.read.counts contains the raw input signal

Impact:

• allows for downstream aggregation of coverage based on dryclean outputs alone as input signal is retained in the outputs
• represents final foreground in same units as input signal