Refs #6352. Adding parameter on peaks source to switch coordinates.
Refs #6352. Getting peak coordinates to switch when asked.

I had to do this as a post-accept check to stop PV from setting the parameter
back to the default when I tried to set the parameter before the accept call.
Found out in the process that the signal connections in MdViewerWidget for
the MultiSliceView don't work anymore. That's why the check signature is now
different. NOTE: Things are currently wired to switch to sample coords. There
is sample code in SplatterPlotView::checkPeakCoordinates() to get things the
right way.
refs #6351. API changes.

Add new method on IMDWorkspace to get any special coordinate system settings. On concrete implementations add setters. Add tests for setters and default values.
refs #6352 #6364. load and save system.
refs #6352. Update existing conversion algorithms
Refs #6352. Add fall down for vtkPeakMarkerFactory case statement.
Refs #6352. Add API call for special coordinate system for MDEW.
Refs #6352. Wiring MDEW PV plugin for special coordinates.
Refs #6352. Switching SplatterPlot view to new property.
Refs #6352. Add API call for special coordinate system for MDHW.
Refs #6352. Wiring MDHW PV plugin for special coordinates.
Refs #6352. Method to switch peak coordinates in SplatterPlot view.
Refs #6352. Merge branch '6352_vsi_peaks_display'