FF loader table count.

This PR addresses a long standing bug within the FF loader.

Previously, the FF loader was not guaranteed to load all fields for an instantaneous dump UMFF, as it replaced the FF fixed header lookup table `table_count` with the value contained within the FF fixed header entry `total_prognostic_fields`.

- [x] Requires https://github.com/SciTools/iris-test-data/pull/34