Document multiple masks for cubes

Working on the STScI JDADF notebooks I noticed users have multiple masks that represent different types of errors. Right now the `SpectrumCollection` object only stores one mask object. It would be useful to be able to handle multiple masks along with

- Maybe be able to select which masks to apply

- Saving and reading multiple masks from file

I would imagine a mask collection object that would do this?

Start by reading the SpectrumCollection object and its current single-mask handling. Clarify the mask collection design, including how users select masks and how multiple masks are saved to and read from files. Done means the object supports multiple masks and both requested workflows are defined and covered.

Written by the indexing model from the issue text.