Homogenize the config keyword that indicates file location

[yymao]

In an effort to make gcr-catalogs more easily installable outside NERSC (#271), we need to allow the root directory of file location to be configurable. The first step is to homogenize the config keyword that indicates file location.

Proposal:

Each catalog config that uses on-disk files must use base_dir to indicate the file location. Additional custom keywords can be used to indicate file names or file name patterns, but the catalog shall not access any file that does not sit in base_dir or its subdirectories.

[wmwv]

I agree with the spirit of this. Which catalog configs currently don't obey this proposal?

But I would like this requirement to be on the catalog config files themselves, but not for the general behavior of specifying custom things in config_overwrite arguments when loading the catalog.

[yymao]

Superseded by #448