Use coords attrs to describe

[aaronspring]

• [ ] lead
• [ ] init
• [ ] member
• [ ] valid_time
• groupby: generated by groupby keyword
• results:
  • 'verify skill': skill from verify
  • 'p': "probability that reference performs better than initialized"
  • 'low_ci': lower confidence interval threshold
  • 'high_ci': power confidence interval threshold
• skill: follows from reference

from bootstrap and verify