After DM-27784, any QuantumGraphs written out with save() now needs to have the extension .qgraph.

Change ctrl_bps code to name the QuantumGraph files *.qgraph.  If any comments refer to the QuantumGraph's storage as a pickle file, fix the comments.  Check the documentation for references to QuantumGraph and pickle file.  For example, if there's an example line for starting from a QuantumGraph, change it to start with qgraph.

The saving of intermediate BPS steps as pickle files should not be changed as part of this ticket.