Avoid storing input sequences as DotPlotMatrix attributes #23
Description
When the sequences are big, storing these sequences (as the s1 and s2 attributes) takes up a nontrivial amount of space -- and we don't really need them around for anything after we've finished constructing the matrix.
It might be better to just store the length, md5 hash?, etc. of the input sequences. This would save space, but admittedly this would technically be a breaking change (the DotPlotMatrix docstring lists s1 and s2 as public attributes).