In `set_rates_on_grids` add ability to write additional attributes

Within the rates_on_grids datasets, it'd be wonderful to be able to add additional attributes to help document what each of the different datasets mean. I can imagine that different analysts could end up storying many different kinds of data here: probability densities over a grid, marginalized rates over a grid, rates over a grid conditioned on certain values of other parameters, etc. To provide a way to document this, maybe set_rates_on_grids can take as an optional argument a dictionary, whose key/value pairs are interpreted as additional attributes to be attached to these datasets?

Happy to work on implementing this if it sounds like a useful idea!