Convert plant_min_temp from C to K for ED2

[Aariq]

Description

Fix for #3110. Adds code to non-exported convert.samples.ED() function to convert plant_min_temp, which is in ºC in BETY, into K, which is the units in ED2.

Motivation and Context

Review Time Estimate

• [ ] Immediately
• [ ] Within one week
• [ ] When possible

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My change requires a change to the documentation.
• [x] My name is in the list of CITATION.cff
• [x] I have updated the CHANGELOG.md.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [ ] All new and existing tests passed.