Add a NuCalibrator class to better interface with the nucal fitting functions

[tyler-a-cox]

#879 adds new functionality to the nucal module which allows the user to fit u and frequency dependent models to sets of radially redundant baselines, but requires some specialized knowledge to generate the set of inputs that go into those functions. We should add a class which takes a few simple inputs, such as antenna positions and a frequency array, and computes radial redundancies and spatial filters, and fits nucal foreground models to the data given some set of visibilities and weights.

[tyler-a-cox]

Addressed in #920