Add MIN_OBS parameter to allow quantifications for loci with spurious reads

Add MIN_OBS (--min-obs) parameter to set the minimum number of reads to be left in a locus when the system is solved. It can be expressed as an absolute number of reads (>1) or as fraction of the observed read count [0..1].

With MIN_OBS > 0 the lp-solver will be forced to provide a deconvolution in all loci with > 0 annotation-mapped reads respectively read pairs. The derived quantifications may be variable in loci with less observations than changes that are necessary to perform the deconvolution.