pgplot has been a dependency for a long time, but have alaways wanted to
ultimately make that optional. This starts that.

the `demo*` codes that do the benchmarking rely on some XB engine
parameter info. Primarily because for ALPACA benchmarking use some data
assumptions to show meaningful results when plotting. But, also because
the library incorporates the network data reorder and the
Fengine/Xengine structures is where that is derived from.