Add ska_fetch script to provide easy command line interface to fetch data

@mbaski - This is an initial go at a command line interface.  Please have a look and give it a whirl.  To do:

- [x] Input validation and help for bad inputs
- [x] Checking for unreasonably large query
- [x] Some examples inline and documentation in the main Sphinx docs
- [x] Validation and tests

This would be accompanied by a launcher script `fetch_ska` that has the environment dependencies built in.  I.e. it'll presumably be different on Windows vs. Unix.  This launcher script might live in some place that everyone already has in their path, for instance in the Greta path.