Adds alternative formula_terms for 'atmosphere_hybrid_sigma_pressure'
Adds alternative formula_terms for 'atmosphere_hybrid_sigma_pressure'

Allows either the use of "a:" or "ap:" in the formula_terms
corresponding to the standard_name of 'atmosphere_hybrid_sigma_pressure'
per Appendix D of the CF Conventions.

aodn/compliance-checker,lukecampbell/compliance-checker,ocefpaf/compliance-checker,DanielJMaher/compliance-checker,ioos/compliance-checker