Point desc_dc2_run2.2i_dr6_v4_object to v4 directory

[yymao]

As reported #604, desc_dc2_run2.2i_dr6_v4_object was an alias of desc_dc2_run2.2i_dr6_v2_object. Indeed these two catalogs are identical; however, for public users, if they downloaded the v4 object catalogs, the newly downloaded files will be stored in a directory with v4 in its path, not v2. As a result, desc_dc2_run2.2i_dr6_v4_object cannot be set as an alias.

[JoanneBogart]

Just to be sure I understand - the issue is that the downloading of, e.g., v4, is done by physical directory, so the file being aliased to would not be downloaded. Is that it?

[yymao]

The physical files are not aliased and will be downloaded from the portal, but the downloaded files will be stored under ^/lsstdesc-public/dc2/run2.2i-dr6-v4/object_dpdd (notice the v4 in the path).

The original config uses alias so its path will be the same as v2, but in reality the path needs to has v4, and that's the issue here.

I just made another commit to switch to based_on, which allows us to use the v2 config but just update the path.

[JoanneBogart]

LGTM