Add progress to Record$cache() and Record$load()

[lazappi]

Getting objects is likely to be slow. It would be nice to see what is happening rather than just waiting.

[lazappi]

#58 does this for Artifact$cache() (for S3). I think for Artifact$load() this would require changes to R/Python anndata packages.

[rcannood]

I think the progress bar for load is simply the cache PB.

This feature was implemented in #58.

[lazappi]

Ok. I guess that works if the file isn't so big that actually reading it from disk takes ages.