clarify dry and fresh weight in trait defintions

[bellerbrock]

Description

Closes #69

Checklist

• [ ] An OBO file generated from an OWL file
• [ ] New terms
  • [ ] The relevant issue has been closed.
  • [ ] Further work is required.
• [x] Term updates
  • [x] The relevant issue has been closed.
  • [ ] Further work is required.
• [ ] CHECKS
  • [ ] New variables have a parent trait ID
  • [ ] New variables have methods and scales
  • [ ] The OBO file has been validated
  • [ ] checked in sweetpotatobase
  • [ ] checked CropOntology