Delete Product CLI Command

[Ariana-B]

Reason for this pull request

Functionality to delete products via the CLI has long been requested.

Proposed changes

• Add datacube product delete CLI command that deletes the specified products and all related datasets. By default, products with active datasets will not be permitted to be deleted, but this can be overwritten with the --force option.

• Misc cleanup in surrounding code; mostly minor changes that should have no user impact

• Mark search_unique_datasets as deprecated since without multiple locations, it doesn't seem to provide anything beyond what search_datasets or search_returning already do.

• [x] Closes #177

• [x] Tests added / passed

• [x] Fully documented, including docs/about/whats_new.rst for all changes