Add delta to SWL to calculate SWCR

[olelod]

If you want to create a wide range for both SWL and SWCR there is a large chance for generating non-physical values, where SWCR is lower than SWL. This PR adds a parameter to the config yaml (defaulted to false), that provides the opportunity to calculate the SWCR from the SWL by adding a delta value.

If swcr_add_to_swcr is set to True the swcr parameter in the config yaml will be treated as a prior distribution for the delta value that will be added to SWL, not as a prior distribution for SWCR directly.

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Contributor checklist

• 🎉 This PR closes #ISSUE_NUMBER.
• 📜 I have broken down my PR into the following tasks:
  • Add swcr_add_to_swl to config_parser
  • Update render_output for relative_permeability
• 🤖 I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR.
• 📖 I have considered adding a new entry in CHANGELOG.md.
• 📚 I have considered updating the documentation.

[wouterjdb]

LGTM 🚀