reading diagonal terms only results in scary numpy error in ms_data_handler

[o-smirnov]

When running a 4-corr MS with diagonal terms only, this line

https://github.com/ratt-ru/CubiCal/blob/master/cubical/data_handler/ms_data_handler.py#L851

crashes out with "Argument 'nparray' has to be a contiguous numpy array." I'm guessing this is because the slicer is selecting correlations 0 and 3 in this regime, and somehow that doesn't play nice with the array shape being relocated. Assigning to @bennahugo, since he added that code (itself a workaround for that godverdamme casacore bug I believe).