Allow user to specify how selective effects combine in overlapping regions

[ArtPoon]

• we assume that selection on non-synonymous substitutions (dN) in overlapping ORFs combine multiplicatively, such that the net effect on substitution rate is w1 * w2 (where w1 is the effect in ORF1 and w2 is in ORF2)
• this is not necessarily the case and there are other ways of combining effects, such as additive (w1 + w2)
• we have to make the assumption that being one or the other applies globally to all sites
• one other method would be a weighted sum, where the user would have to specify an "importance factor" for each ORF

[ArtPoon]

@SandeepThokala please coordinate with @horaciobam