Describe the bug

When the RhessiLoader got moved to extern some of the methods/properties got renamed or modified.

Thanks to @ianan for pointing this out. The "legacy" code should continue to work even if the loader has been moved to extern

Here's a list of things that we need to support. Most can be fixed with simple patches. The old names should probably be given deprecation warnings.

• Passing in the spectrum and SRM FITS files to Fitter doesn't work for RHESSI
• the property data2data_minus_background got renamed to background_subtract
• Time-related properties
  • start_event_time
  • end_event_time
  • start_background_time
  • end_background_time
  • Frankly--I think these should be updated in pairs, but we can support the individual getters/setters
• Spectrum energy bounds don't get respected when set near the edges
  • Not sure if this was ever the case, but we can support it here if desired.

I think those are the changes between the previous RHESSI loader and the current. I'll open a PR shortly which addresses these.