Add unit tests for virtual sensor calculations
Add unit tests for virtual sensor calculations

This checks whether the machinations of DataSet + SensorCache deliver
the values expected from katpoint. To help this along, create a minimal
DataSet subclass that survives basic data set manipulations (i.e.
select and print) and supports all the default virtual sensors.

This pushed test coverage for dataset.py from 16% to 58%.

ska-sa/katdal