Use run_variables instead of runs in Damnit.table()

[JamesWrigley]

This is a lot faster because we can filter out blobs, which we don't want to load anyway. The PR is incomplete, I'll be away for the next week but if anyone else wants to finish it off feel free :)

Breaking changes:

• The run number is now set to the index so you can .loc the table by run number.
• The proposal column is removed, I think it's usually not needed.
• None values in the old table where cells didn't have any data are now set to pd.NA.

TODO:

• Add missing columns to the dataframe (see code comment).
• The code currently removes the proposal column under the assumption that it doesn't change and the user probably isn't interested in it, but we should add an argument to keep it in case it's necessary in the future.
• Add tests.