Extension alignments longer than 2000 bp

[marcelm]

Sometimes ssw_align() is asked to compute an alignment for a reference sequence longer than 2000 bp here: https://github.com/ksahlin/strobealign/blob/8d3ef7e113abe9159a38343d970e864fc5afaab4/src/aln.cpp#L35-L43

This gets triggered in the Drosophila test dataset for read @SRR6055476.50009. The chunk needs to start a couple of reads earlier. Putting reads @SRR6055476.50004 to @SRR6055476.50009 (paired end) in paired-end FASTQ files reproduces it reliably.

As discussed in #168