Make equal sequence length an invariant of Alignment.
Flake8 changes.
Updated Alignement.is_valid() doctest.
Added change description for issue #670: Make equal sequence length an invariant of Alignment.
Removed Alignment.is_valid.  Return AlignmentError when Alignment sequences are not the same length.  Fixed SequenceCollection equal/not-equal unit tests.
Updated PhylipFormatError message in _alignment_to_phylip when object is not valid now that Alignment sequences are guaranteed to have the same length.
Fixed typos based on @jairideout comments.
Merge pull request #694 from laurentluce/alignment_sequence_length

Make equal sequence length an invariant of Alignment.