Improve mass plane requirement

[andlessa]

Build a cleaner and consistent way of relaxing the data grid requirements. Allow for mass points to be slightly off the grid (1 GeV?).

• [ ] Remove the "gradient" test (still used?)
• [ ] Use the find_simplex tolerance option throughout.
• [ ] Make it consistent with the validation tools in smodels-utils (getXYValues,...), so the validation and file creation codes do not break