add reset functionality for attributes

[DaJansenGit]

For attributes, for which a __get__ via element.attribute (e.g. tz.lighting_power) has been performed before and thus are already calculated, the status will be STATUS_AVAILABLE. Those attributes won't be recalculated again if the get` method is called again. This behaviour is on purpose and totally fine, but in some cases (e.g. enrichment) we want to reset those attributes, to make sure that the enrichment works properly.

Therefore we need a reset option for attributes.