renamed:    delphin/mrs/mrs.py -> delphin/mrs/basemrs.py
deleted:    dmrs2mrs.py
deleted:    mrs2dmrs.py

Added top-level mrs.py script with a 'convert' command:
  python mrs.py convert --from simplemrs --to dmrx --pretty-print infile.mrs

Added default delphin.mrs package-level classes in delphin/mrs/__init__.py,
and modified delphin/mrs/{dmrx,mrx,simplemrs,tests/mrs_test}.py files
accordingly.