unifies how we extract samples from multivalued functions

[reedessick]

This patch fixes a bug in process2samples associated with extracting the values from multivalued functions.

• [x] implement selection_rule within process2samples
• [x] implement branches_mapping within process2samples, including the use of default values
• [x] unify the implementation within plot-process and process2quantiles with this approach
  • [x] update samples.process2quantiles to delegate to samples.process2samples as needed
  • [x] update command-line options for plot-process and process2quantiles
• [x] TEST all implementations!