Class methods to print allowed metadata keys and available "rates on grids" datasets

In addition to the get and set methods for accessing/saving specific metadata and rates over grids, it would be helpful to have corresponding print methods to enable easier exploration of the different datasets available. In particular, things like the get_rates_on_grids method requires users to know in advance the key they want, which they may not if handling a file created by a different author. One could always open the popsummary result as a standard hdf file and explore the datasets manually, but it might be nice to offer a built-in way to do this.

For example, maybe there could exist something like print_rates_on_grids() and print_metadata() that print the available keys in the hdf file's attributes and rate datasets?