Add (un)packing for {Velo,ST}Cluster containers
These differ from the existing {Pack,Unpack}Clusters algorithm in that
they each input a (packed) {Velo,ST}Cluster container and output a
(un)packed location filled with a single (packed) {Velo,ST}Cluster type.
The existing algorithm takes a Track container and creates a single
packed location containing all *Cluster types, and then unpacks this to
non-configurable default locations.
We need more flexibility than this in Hlt2, specifically for Turbo{,SP,++}, where we don't necessarily know all the input Track containers at configuration time (unlike in the Stripping, where you just point PackClusters to Rec/Track/Best).
I'm still performing some tests, but haven't marked this as WIP as I'd like it compiling in the nightlies.
Depends on !818 (merged).