Add unit of measurement

[robmarkcole]

If this isnt included the state is treated as a string, not numerical

[robmarkcole]

Hmm adding the following did not change this:

    @property
    def unit_of_measurement(self):
        """Return the unit of measurement."""
        return ""

[robmarkcole]

OK the unit cannot be an empty, the following adds a graph:

    @property
    def unit_of_measurement(self):
        """Return the unit of measurement."""
        return "targets"

[robmarkcole]

done