Extracting off-targets is at risk of crashing if only one sequence is processed

[jakeb1996]

The extract off-target sites utility will crash if there is only one FASTA sequence provided. This leads to only one intermediate file existing; these intermediate files are sorted and merged. Importantly, the sort is successful but the merge is not.

I do not believe there will be an issue when the input(s) are either: (1) multiple FASTA files or (2) a single multi-FASTA file.

The crash is caused by a variable being referenced before assignment:
Line 191 exists outside of the while loop, and therefore, mergedFile may never be declared.

Crackling/src/crackling/utils/extractOfftargets.py

Line 191 in bb50af9

shutil.move(mergedFile.name, fpOutput)