Basic CLI structure

[jeromekelleher]

As discussed in various other places (see #53 and linked issues) a CLI will be useful.

I suggest we implement this as python -m sgkit <sub-command>, using click.

Initially we'd probably want:

• sgkit info (list the contents and sizes of variables)
• sgkit convert-vcf <vcf_path>

We might want to support other stuff like (say) sgkit rechunk.

We'd probably have some common options for connecting to a Dask scheduler, which would be good to think about ahead of time.

Thoughts?

(I guess we can close this issue once we have the first PRs with the initial CLI)