add more unit tests

[dave-doty]

This is a bit vague. Unfortunately there is very little unit testing in the library now. One reason is that testing the search algorithm itself is difficult since there is so much randomness. However, parts of the library are perfectly deterministic and can be tested, for example the DNA sequence generation used by the NumPy constraints (largely from the submodule dsd.np).

Even some parts of this that are random can be unit tested. For example, the constructor for DNASeqList that generates a random subset of sequences exactly a given Hamming distance from a given sequence (through the parameters hamming_distance_from_sequence and num_random_seqs) can be tested to ensure that every sequence it creates is the correct Hamming distance from the given sequence.