Fake dataset generator

[iancze]

Is your feature request related to a problem or opportunity? Please describe.
To examine and test priors, it's helpful to have fine-grained control over the mock-dataset generation process, without needing a CASA dependency for simobserve.

Describe the solution you'd like
Given an ImageCube layer (it will be up to the user to construct this properly, with GridCoords object), and a set of u, v, points, and corresponding weights, produce a set of complex visibilities.

Provide both the "noiseless" visibilities as well as the mock dataset.

This can use the NUFFT dataconnector once #17 is completed.

[iancze]

Closed by #111