sortmerna + sumaclust added to the open reference OTU picking pipeline

I've changed the argument verification code slightly to ensure --sortmerna_best_N_alignments can be set (to something other than 1) iff --sortmerna_tabular is set. 

By default, --sortmerna_best_N_alignments=1 is passed to sortmerna, with or without outputting alignments as we require at least 1 best match in order to build an OTU map.