Nexrad Level2 different range resolution not taken into account

[kmuehlbauer]

See #259 for an example case.

For sweep number 4, 5, 6 REF is 1000m range resolution whereas VEL and SW is 250m resolution. In the pyart reader the lower resolution data is interpolated to the higher resolution data to fit into the radar object.

Unfortunately this was overlooked on implementation time that this not only happens between sweeps (which would not be a problem here) but also within a certain sweep. So how to resolve this issue for xradar?

I'd really like to avoid this interpolation/upsampling, to not change the source data. We could just add another sweep/group in these cases. WDYT @openradar/xradar?