Find and return a :class:`~dossier.fc.StringCounter` at
    `feature_name` or at `DISPLAY_PREFIX` + `feature_name` in the
    `fc`, or return None.

def get_string_counter(fc, feature_name):
    '''Find and return a :class:`~dossier.fc.StringCounter` at
    `feature_name` or at `DISPLAY_PREFIX` + `feature_name` in the
    `fc`, or return None.

    '''
    if feature_name not in fc:
        feature = fc.get(FC.DISPLAY_PREFIX + feature_name)
    else:
        feature = fc.get(feature_name)
    if isinstance(feature, StringCounter):
        return feature
    else:
        return None