refs #10365. Add regression test.

No fix yet. But helps with TDD.
refs #10365. Use numeric axis not y values!
refs #10365. Missing init call.

We don't need a managed algorithm. Not sure how this ever worked! Later on, I'll create a MantidPlot test for the Rebin2D lineviewer functionality to stop this from falling over again.
Merge branch 'feature/10365_slice_viewer_crash' into develop