Rewrite save rules in terms of GribMessage

[pp-mo]

All the grib2 load rules in iris_grib._load_convert are written in terms of the sectionally-structured GribMessage object. But all the save rules (iris_grib._save_rules) use the gribapi (and/or eccodes) instead

Technical Debt summary:

• fix:

  • extend GribMessage to add 'create from template' and 'write to file'.
  • rewrite existing save rules in terms of GribMessage, and rearrange sectionally (like load rules)
  • costs of change:
    • different from load rules means more to learn, hard to read
    • testing is especially more complicated

• costs of the legacy support :

  • code is rather exposed to changes in the ecmwf APIs
  • confusion from the different types of "message" in our docs
    • for devs. especially, in our the testing code
  • existing save rules are less tidy, as you don't get the benefit of the sectional division of the GribMessage object :
    • unlike the load rules, where appropriate data sections are passed, which is neat + clean
    • this also complicates testing with elaborate schemes to mock out gribapi operations

---

To fix:

• make it possible to create a GribMessage from a gribapi template, and save it to a file.
• rework the existing rules to work on GribMessages instead of a gribapi handle.
• refactor the tests likewise (including various test classes mocking out message access)