Improve code deciding which NetCDF variables to import into the `variable` table

See #5 for the whole picture. Currently the code deciding which variables to import into the database is really brittle. What should actually be done is taking the difference between `Dataset.variables.keys()` and `Dataset.dimensions.keys()` while additionally skipping special variables which are not dimensions like e.g. `'lat'`, `'lon'`, `'rotated_pole'` and `'time_bnds'`.