Minor improvements to UpdateIRLS stopping criteria

[santisoler]

Summary

Apply some minor improvements to the stopping criteria method of directives.UpdateIRLS. Ditch the hard-coded 1e-12 value that gets added to the f_old to avoid division by zero: assign f_change to np.inf instead. Remove the absolute value computation while comparing 1 - chi_factor with the misfit_tolerance. This way we avoid ending the inversion with a chi factor above the 1.0.

PR Checklist

• [ ] If this is a work in progress PR, set as a Draft PR
• [ ] Linted my code according to the style guides.
• [ ] Added tests to verify changes to the code.
• [ ] Added necessary documentation to any new functions/classes following the expect style.
• [ ] Marked as ready for review (if this is was a draft PR), and converted to a Pull Request
• [ ] Tagged @simpeg/simpeg-developers when ready for review.

Reference issue

What does this implement/fix?

Additional information

TODO

• [ ] Enable users to use np.abs when checking for the chifactor within tolerance. Basically, allow users to stop the IRLS even if the chifact is slightly higher than the target.