dssp style secondary structure assignment

modules/mol/alg/doc/molalg.rst

modules/mol/alg/pymod/CMakeLists.txt

modules/mol/alg/pymod/export_sec_structure.cc

modules/mol/alg/pymod/wrap_mol_alg.cc

modules/mol/alg/src/CMakeLists.txt

modules/mol/alg/src/sec_struct.cc

modules/mol/alg/src/sec_struct.hh

modules/mol/alg/tests/CMakeLists.txt

modules/mol/alg/tests/test_partial_sec_struct_assignment.cc

modules/mol/alg/tests/test_sec_struct.py