Need a way to check sample completeness without downloading anything

[dkirkby]

Perhaps a --local-inventory option to bossfetch? The idea is to be able to issue a command like this:

% bossfetch --local-inventory sky.dat
1234 of 233807 observations are locally mirrored.

[dkirkby]

There is already a --dry-run option that stops after building the list of remote files, so it would be enough to add a loop over these files that reports the number of files that already have corresponding local mirrors before exiting.

[dkirkby]

Fixed in #37