'Adds a field to the device
Arguments:
name     -- Name of the field
typename -- Type of the field (numeric, boolean, dateTime, timeSpan, string, enum)
unit     -- [optional] only applies to "numeric". Unit for the field.
dataType -- [optional] only applies to "enum". Datatype for the field.'

def _add_field(self, name, typename, unit=None, dataType=None):

    self.fields[name] = {'type': typename, 'unit': unit, 'dataType': dataType}
