Return None/NaN if no new data is available

[dschick]

Hi @tox1c90

Right now this TangoDS always returns the last measured value from the PEM as read-back value of the power.
I think, that this is maybe not the correct way of implementing it.

I would suggest, that the attribute return None or NaN (possibly the first) if no data is available.
This would make it consistent with other powermeters such as the Coherent LabMax or ThorlabsPM100 series.

Is there an easy way of checking the number of new data readings from the API?