shared-cache - enable shared-cache support in SCR

SCR can direct the application to write dataset files to subdirectories within a cache directory. SCR also stores its redundancy data in these subdirectories.

**Question**: Should it be considered an error to configure redunancy schemes when cache is shared?

To construct the full path of a cache directory, SCR incorporates a cache base directory name (`SCR_CACHE_BASE`) with the user name and the allocation id associated with the resource allocation.

The cache directory name is **_currently_** derived from the concatenation of the cache base directory (`SCR_CACHE_BASE`), the user name running the application, and the job scheduler resource allocation id. This presents a name collision problem when the cache is on a shared file system.

This ticket proposes that the cache directory name should also have the MPI rank numbed appended to the name above.

**Question**: Should we just append this in general? Or only when the cache is on a shared file system, which begs the question of how SCR can determine when/if the file system is shared. My vote is to simply append the rank number as a general rule after the session id.