Refs #9492. Set set correct Y unit for the MantidMatrixCurve

It was being set to getAxis(1)->unit() before, which isn't really a Y
axis, but rather "Z" axis, used in 2D plots. "Z" axis was actually
different in a fitted ws (hence the warning),  but it isn't used for 1D
plots, so it shouldn't matter.
Refs #9492. Add a few comments
Refs #9492. Compare Y units by caption instead of unitID

Y units are always of type Label, hence unitID will always be the same
Merge branch 'bugfix/9492_fit_units_warning' into develop