Merge machinery for plotting form factors with standard test and plot facilities

• Symmetric plot range (!760 (merged))
• Axes units deg, not rad (!758 (merged))
• Rename sim_ -> Sas (!757 (merged))
• Restore normalization to maximum value (!763 (merged))
• Import ba_plot as bp in all examples (!765 (merged))
• Pass detector size as command-line argument; simplify persistence tests (!773 (merged))
• Use new multiPlot in example PositionVariance (deferred to #259 (closed))
• Merge bornplot with ba_plot (!773 (merged))
• Command-line argument to generate pdf instead of showing plot (!773 (merged))
• Cover ff scripts by CI tests (!773 (merged))
• CMake target to rebuild figures (!773 (merged))
• Review all ff plots (deferred to #258 (closed))