dss-opt-1.0.5

Mainly improvements to make building the Julia and Python interfaces easier

• move force constants to na.h
• rename random_seq to random_useq, xstr_to_seq to xstr_to_useq
• new variants of functions that don't exit on error: vienna_to_pairs, pairs_to_vienna, parse_seq_constraints_hard
• new functions: xuseq_to_str, useq_to_str
• add some unittests: make unittest and make unittest-valgrind
• run all checks with valgrind: make check-valgrind