Pass top level compute arguments through to compute inside `find_objects`

[GenevieveBuckley]

Suggestion from Marvin, about how we might improve the find_objects function https://github.com/dask/dask-image/pull/240#discussion_r681072020

Hmm I'm wondering whether arguments to the top level compute call, such as the chosen scheduler, would need to be forwarded to this call. Maybe there's a way to access these here?

Follow up to https://github.com/dask/dask-image/pull/240