Prime editing alignment params

[kclem]

Adds two parameters to control alignment of pegRNA components: --prime_editing_gap_open_penalty and --prime_editing_gap_extend_penalty.

CRISPResso checks to see whether the pegRNA spacer and extension sequence are in the correct orientation, but sometimes they could align in the incorrect orientation with a higher score (e.g. via insertion of multiple gaps, whereas a single long gap would be preferred). Introducing these two parameters allows users to adjust the alignment parameters specifically for these prime-editing checks without adjusting the global alignment parameters which will be applied to reads that are aligned to the WT reference/prime-editing reference sequences.

The new prime_editing_gap_open_penalty is set to -50, a higher gap open penalty than the default needleman_wunsch_gap_open penalty (-20). This commit breaks backward-reproducibility, but mostly in the checking of pegRNA component orientation - so previously some CRISPResso runs would have failed and produced an error, but now they will (hopefully) succeed. To achieve complete backward reproducibility, add the flag --prime_editing_gap_open_penalty -20 to runs.