Add caching to cmip6 ingestion

This adds a global cache for holding cmip6's table_id to frequency and realm. It also adds a small utility to create some test cmip6 netcdf files with appropriate attribute data to be able to check this code path in our tests. The resulting files are committed so it doesn't need to be run frequently, only when test cases need to change.

freva/src/drs/metadata.rs

@@ -395,8 +395,7 @@ lazy_static! {
     /// because any table_id should always have the same frequency and realm values.
     ///
     /// I'm not sure if this is a property of CMIP6 or just a coincidence but seems to hold for all current possible
-    /// values in the controlled vocabulary. See
-    /// [here](https://gitlab.dkrz.de/freva/py-cmorlite/-/tree/master/cmorlite/cmor/CMIP6) for how table_id
+    /// values in the controlled vocabulary. See [here](https://github.com/PCMDI/cmip6-cmor-tables) for how table_id
     /// determines frequency and realm.
     ///
     /// This is done as a global cache right now just because performance shouldn't be a large concern as we expect far