Fix file_sse_pool and ss; correct jufo extension; simplify interval; Fix and add tests.

Fix file_sse_pool to only extend the cc_sequence when reading sse lines to correctly read .pdb files.
Fix ss to return sequence of correct length.
Change jufo extension from .jufo to .jufo9d_ss.
Simplify interval and sequence_interval less*() and equal*().
Fix tests for interval and file_sse_pool; add tests for file_qs.
Add data files for tests.