add GetMatchingBackboneViews to AlignmentHandle

modules/mol/base/pymod/export_entity_view.cc

modules/mol/base/src/entity_view.hh

modules/seq/base/doc/seq.rst

modules/seq/base/pymod/const_seq_list_export_def.hh

modules/seq/base/pymod/export_sequence.cc

modules/seq/base/src/alignment_handle.cc

modules/seq/base/src/alignment_handle.hh

modules/seq/base/src/impl/sequence_impl.hh

modules/seq/base/src/sequence_list.cc

modules/seq/base/src/sequence_list.hh