Workaround Multiply (for scaling) which does not propage Dx

- new method scaleWorkspace which reinserts Dx values for point data
- deleted MantidDataObjects/WorkspaceSingleValue.h from Multiply and Divide
- introduced new class variables m_scaleFactor and m_errorScaleFactor; those values will now be initialised by 1.0 (the default value) and not 0.0 as before.
- deleted unused tool-tip texts (Optional.)
- ConjoinXRuns output workspace property is a Workspace and not a MatrixWorkspace -> dynamic pointer cast
- unit tests for histograms pass
- MantidPlot executes for point data example from unit test
- unit test for point data fails (SortXAxis, Python algorithm, not registered!)

Refs #22197