Merge pull request #13958 from mantidproject/13897_ISIS_ET_Background_in_data_range

As far as the validation goes this PR fixes the problems. However, When plotting I found that once you have removed background in the range 65000-75000 for example, if you try to adjust the range (65000-65001 for example) then the plot is not updated and the background associated with the previous range will be plotted instead. I would say make a separate issue for this as it is beyond the scope of this one.