Speed up from O(n^2) iteration over all left/right primer hits in OffTargetDetector._to_amplicons

[ameynert]

OffTargetDetector._to_amplicons uses itertools.product over the left and right primer hits and evaluates each of them in a loop to identify valid left/right hit combinations for the pair.

Suggestion:

1. Get hits by primer sequence from mappings_of
2. Split into left and right hits
3. Group hits by refname
4. For each refname, split into left +, right -, left -, right+ hits
5. Build amplicons from left + and right - (amplicon strand +), left - and right + (amplicon strand -)

Building amplicons - dealing with only positive & negative in correct orientation and known to be on the same reference, with known amplicon strand

1. itertools.product over positive & negative primer hits
2. Check product size against min/max amplicon size range
3. If in range, return the product hit as positive start to negative end with the known reference name and amplicon strand