fix standard deviation sheet

[dodu94]

Description

The standard deviation sheet was computed as the absolute difference between the results (reference-target) divided by the relative error value of the reference. The actual quantity of interest here is instead the absolute difference divided by the product of ref error times ref value in order to divide by the absolute error. This small PR fixes this issue in the Sphere and SphereSDDR benchmarks.

Type of change

Please select what type of change this is.

• Bug fix (non-breaking change which fixes an issue)
• New benchmark
  • Non-breaking change which entirely uses exisiting classes, structure etc
  • Breaking change which has implemented new/modified classes etc
• New feature
  • Non-breaking change which adds functionality
  • Breaking change fix or feature that would cause existing functionality to not work as expected