add seq.Match and use it in MergePairwiseAlignments

modules/seq/alg/src/merge_pairwise_alignments.cc

modules/seq/base/doc/seq.rst

modules/seq/base/pymod/export_sequence.cc

modules/seq/base/src/sequence_handle.cc

modules/seq/base/src/sequence_handle.hh

modules/seq/base/tests/test_sequence.cc