A `checkbox` with no enumerated values results in an error

[gaurav]

Attempting to translate a REDCap CSV file with a checkbox field without any enumerated values results in a mysterious looking error:

Traceback (most recent call last):
  File "/Users/gaurav/Developer/heal/heal-data-dictionaries/venv/bin/vlmd", line 8, in <module>
    sys.exit(vlmd())
             ^^^^^^
  File "/Users/gaurav/Developer/heal/heal-data-dictionaries/venv/lib/python3.12/site-packages/click/core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/gaurav/Developer/heal/heal-data-dictionaries/venv/lib/python3.12/site-packages/click/core.py", line 1078, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "/Users/gaurav/Developer/heal/heal-data-dictionaries/venv/lib/python3.12/site-packages/click/core.py", line 1688, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/gaurav/Developer/heal/heal-data-dictionaries/venv/lib/python3.12/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/gaurav/Developer/heal/heal-data-dictionaries/venv/lib/python3.12/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/gaurav/Developer/heal/heal-data-dictionaries/venv/lib/python3.12/site-packages/healdata_utils/cli.py", line 205, in extract
    data_dictionaries = convert_to_vlmd(
                        ^^^^^^^^^^^^^^^^
  File "/Users/gaurav/Developer/heal/heal-data-dictionaries/venv/lib/python3.12/site-packages/healdata_utils/conversion.py", line 268, in convert_to_vlmd
    data_dictionary_package = choice_fxn[inputtype](
                              ^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/gaurav/Developer/heal/heal-data-dictionaries/venv/lib/python3.12/site-packages/healdata_utils/transforms/redcapcsv/conversion.py", line 139, in convert_redcapcsv
    targetfields = gather(sourcefields)
                   ^^^^^^^^^^^^^^^^^^^^
  File "/Users/gaurav/Developer/heal/heal-data-dictionaries/venv/lib/python3.12/site-packages/healdata_utils/transforms/redcapcsv/conversion.py", line 91, in gather
    targetfield = typemappings[sourcefieldtype](sourcefield)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/gaurav/Developer/heal/heal-data-dictionaries/venv/lib/python3.12/site-packages/healdata_utils/transforms/redcapcsv/mappings.py", line 216, in mapcheckbox
    for val,choice in choices.items()
                      ^^^^^^^^^^^^^
AttributeError: 'str' object has no attribute 'items'

This should be improved to indicate why the error occurred (but there are no enumerated values for this field), but it may also be a good idea to allow this -- otherwise, we have to add a fake value (e.g. NA, NA) so that the file can be transformed.