Improved compatibility checks with to_/from_yaml

[dafyddstephenson]

Currently, roms-tools warns about any version mismatch between the version recorded in the yaml file and the current version of roms-tools, even if the resulting datasets do not change between versions.

I propose we

• Keep the current system for all versions below 1.6.0 (or whatever version is current when this gets addressed)
• Going forward, use pytest to create a series of netCDF files from a chosen set of yaml files and compare their hashes against a known manifest. If there is a mismatch between the most recent hashes recorded in the manifest (i.e. it has not been updated), the test should fail.
• Use this manifest internally to determine whether the user's version is truly incompatible with the current version before issuing the warning.

@NoraLoose what do you think?