- add: xls2xform.convert for library users to call pyxform without
  needing to use files (accepts bytes/file handles/strings)
  - accepts markdown input since this is widely used by pyxform
  - accepts dict to avoid needing to use internal funcs that may change
- chg: avoid writing to files unless validate=True (for ODK Validate)
  - also avoid assuming any files were written, e.g. missing_ok=True
- chg: move xls/x_sheet_to_csv, sheet_to_csv from utils.py to
  xls2json_backends.py because they are backends for csv input.
- chg: move md_to_dict from test directory into xls2json_backends.py
- chg: refactor pyxform_test_case.py to use xls2xform.convert only,
  instead of internal funcs associated with md_to_dict, so that the
  existing tests check API stability e.g. file types, dict input, etc.