Limit memory usage of alignment

[Martinsos]

Right not alignment eats up quadratic amount of memory. For big sequences this will be a lot of memory, which makes swimd unusable for finding alignment of very large sequences.
I should do something similar to Hichbergs algorithm, in order to limit memory usage. It will make alignment slower for some factor, but that is a trade-off we can not avoid.
It would be nice if user could specify this memory limit.