Steiner porting

[brettviren]

This is a meta issue to keep track of steiner related porting.

Some things I know already we will need:

• [-] A way to "clone" a cluster so that it has a clone of the Cluster and chlid Blob facade data as well as underlying clone of PC tree nodes and node values.
• An ensemble visitor to loop over cluster (CreateSteinerGraph)
• A per Cluster "wrapper" class that can be loaded up with data and methods akin to the Steiner-related portion of PR3DCluster (Steiner::Grapher)
• A way to provide various "service" components to each wrapper and such that any cloned instances can also get them. (Grapher::Config struct)
• Examples of and support for graph filtering (see reduce() and weight_threshold() in GraphAlgorithms`)
• Ability to "slice" PC array or form "subset" of PC Dataset given collection of indices.
• Support to form monolithic PC from scoped view (actually already had this, see flat_coords() and the more general flat_pc() of ScopedBase.
• ....