Modifying max_sequence_length method of Alignment.

Changing the max_sequence_length method of sate.alignment.Alignment
so that it ignores gaps (i.e., '-' characters). Thus, it now does
as its name implies and returns the length of the longest
unaligned sequence.
Adding test of Alignment.max_sequence_length

Adding AlignmentTest.testMaxSequenceLength which tests to determine
if the sate.alignment.Alignment.max_sequence_length method ignores
gap characters (i.e., '-').