Generate ext in p. descriptions

[martijnvermaat]

The description extractor cannot conclude that the variant protein is an extension of the original, because it does not know the DNA sequence of the 3' UTR (by design). Instead, it will probably yield an insertion at the end (or something similar).

A possible way of fixing this would be to add knowledge of the DNA sequence to the extractor algorithm, but this is rather at odds with its basic design (i.e., that the variant can be generated solely from the original and variant sequences in a deterministic way).

After discussion with @jfjlaros and @jkvis is was decided we will try to implement this by adding an optional hint to the description extractor that the variant is a possible extension (i.e., because the stop codon was changed). This hint is not passed to the core algorithm, but used by the higher-level interface generating the HGVS description string.

This hint can be used by the caller when the variant on DNA or cDNA level is known, which is the case for Mutalyzer.

The original todo note for this can be found here.