Fix for import_mothur cutoff and missing taxa

When using import_mothur provided cutoff values were ignored due to
missing return(cutoff) argument. When using import_mothur provided
cutoff values were ignored due to missing return(cutoff) argument.
Additionally the last two OTUs were missing because
as.integer(rawlines[2]) gives back the number of OTUs but rawlines
contains two more elements (cutoff and number of OTUs).
Additionally the last two OTUs were missing because
as.integer(rawlines[2]) gives back the number of OTUs but rawlines
contains tow more elements (cutoff and number of OTUs).