Feature: a simple config harness to evaluate changes to MFDs

[chrisbc]

SRM need to understand the impacts of various MFD changes without the full inversion/diagnostics cycle.

Add we've identified two changes needed in the main MFD plot to assist with evaluations.

Compare:

• OPENSHA http://nzshm22-static-reports.s3-website-ap-southeast-2.amazonaws.com/opensha/DATA/SW52ZXJzaW9uU29sdXRpb246[…]ources/mfd_plot_Total_Target_MFDs.png with
• Toshi http://simple-toshi-ui.s3-website-ap-southeast-2.amazonaws.com/InversionSolution/SW52ZXJzaW9uU29sdXRpb246MTM2MTEuMGI4U1lK/InversionSolutionMfdTab

In the opensha report we want

• [x] add the truly-onfault target (targetOnFaultSupraSeisMFD_SansTVZ + targetOnFaultSupraSeisMFD_TVZ)
• [x] adjust the the target gridded to be purely the gridded seismicity (trulyOffFaultMFD.all) . I think is is currenlty Gridded + SubSeismo

[chrisbc]

FYI @earjcr1 @mcg-gns This story supports #72