Resolve "In `set_rates_on_grids` add ability to write additional attributes"

Closes #3 (closed)

@zoheyr-doctor @thomas-callister Unfortunately, hdf5's "attrs" doesn't support dictionaries. To get around this, when setting optional attributes in set_rates_on_grids, I've made it so that you pass two additional arguments: a list of attributes, and a corresponding list of attribute keys. Each key and attribute pair then gets iteratively stored as an individual "attrs" in the results file. They can be retrieved by giving a list of desired attribute keys to get_rates_on_grids. Please have a look at the updated tutorial for an example. Let me know if you're satisfied with this implementation and I can merge it.

I've also added the option to return the parameter list with get_rates_on_grids.