Ribosome Binding Sites motifs

[camilogarciabotero]

This PR implements a new feature that detects and scores Ribosome Binding Sites (RBS) Motifs given an ORF. It is based on the possible spacers (offset) of different kinds of DNA motifs:

Spacer Range	Representative RBS Motif(s)
3-4 bp	GGA, GAG, AGG, AGxAG, AGGAG
5-10 bp	GGAG, GAGG, GGxGG, AGGAGG
11-12 bp	GGA, GAG, AGG, AGxAG, AGGAG
13-15 bp	GGA, GAG, AGG, AGGA, AGGAG

Since there is some overlap we implemented the search of motfs in three different windows:

• Window Checking A:

                 -10    -3  |-> start codon
               ...|......|..ATG...

• Window Checking B:

           -16        -5    |-> start codon
         ...|..........|....ATG...

• Window Checking C:

       -20      -11         |-> start codon
       .|........|..........ATG...

And add more motifs and a basic coding values based on the phannotate idea.

This lead to implement a simple RBS struct to store the motif and other information and three methods related to the detection, scoring and a new coding decision rule (_findrbs, orf_rbs_score, ribsome_binding_site_decision_rule).