Introduce switchable **opt-out** new-style GRIB1 loading

[trexfeathers]

New-style introduced in #489.

Opt-out because we need to force users to experience the new alternative. We don't expect any changes, but we need to get this exposed so we can be sure.

Possible implementations

• Environment variable - would allow switching without editing code
• Global Python variable - flexible and simple (but not thread safe)
• Thread-safe context manager (e.g. iris.FUTURE) - would allow both behaviours within a single script

Deprecation

If the user uses the switch(es), there needs to be an appropriate DeprecationWarning because the switch(es) will not be there forever. Will be removed in X number of releases.