Drop _FillValue from NetCDF product

[forman]

In the LC NetCDF product 1.6.1, the attribute _FillValue=0 is used on the lccs_class variable. We currently have an unsolved issue in xarray which causes a conversion of any variable with a _FillValue into a float64. The reason is that so NaN can be used instead of _FillValue and various numpyand scipy functions become applicable that can filter out NaNs (e.g. nanmean).

According to a clarification on _FillValue, missing_value, valid_xxx, the use of valid_min=1 and valid_max=220 is sufficient and therefore the _FillValue could be simply dropped as 0 is not the only missing data / no data in the possible uint8 range.