Introduce a new command to create additional cache copies within a grid out of existing ones

It is required to have the possibility to create new copies of virtual tapes out of existing (cache) copies in the same grid.

As of today, it is not possible to create additional cached copies once the CCP is correct. TS7700 does not distinguish then if the copy is in cache or on physical tape only. This is not beneficial if an installation has a disbalance of cached copies over the years. Whenever applications are dependent on the short mount times from cache copies, any outage (break, microcode upgrade, lifecycle) of one cluster in a grid may impact the mount times on the remaining cluster(s) negatively.

The usage of external tools like PRESTAGE is taking too long and not very user friendly, plus requires RACF changes, plus requires APF authorized library access.

The copy of new cache copies must be made possible e.g. by providing a LI REQ <dist-lib>,PRESTAGE,<source-lib> or equivalent command, with the following functionality : 

When dist-lib = source-lib, a cache copy is created from the same cluster forcing a RECALL (from physical tape). When dist-lib <> source-lib, the additional copy is created from the existing one on source-lib. If there is no copy on source-lib or source-lib is omitted, the next available cache copy is taken as source. When no cache copy is available, a physical mount is always initiated.

Rational: for large numbers of cache copies to be created on one cluster, the usage of external tools like PRESTAGE is clumsy and inefficient, takes too long.