Add an array for tracking visibility noise estimates

[plaplant]

There are several use cases where it would be nice to have an array that captures and estimate of the visibility noise. In general, this is a quantity which can be the size of the data, and real-valued. We may want to support an optional data-like array that contains this information. Some of the file formats support such a quantity to various degrees, so we should think about the best way to handle it for different cases.

[bhazelton]

Should warn that it will be dropped when writing to a file format that doesn't support it.