Compute custom CL-motifs

1. Detect crosslink sites on the input data using the basic version of PureCLIP.
2. Learn CL-motifs by running DREME (Bailey et. al, 2011) with the parameters -norc -k 6 -m 4 on 10-bp windows spanning the called input crosslink sites, while using 10-bp windows 20 bp upstream and downstream as the control.