Change Behavior: Should return NA when length not a multiple of 3

[mikegilchrist]

Right now we get a warning if a gene isn't a multiple of 3 nucleotides.
However, a phi value and other stats returned for these genes.
Instead, ROC should return "NA"

[acope3]

Alternatively, we shouldn't even store these in the genome object, so nothing will be output. Also could put this on the user, i.e. make clear in the documentation that the user is responsible for inputing appropriate CDS sequences (although we will tell them which sequences are problematic).