Add option param_defs for read_grid()

[pollyaschm]

@adeckmyn , as brieftly discussed, I am adding the change that made me pass the option param_defs on to read_grid().
I did this change for the read_grid() of the deterministic model only.
In my case I use this to make harp add the three variables rain+snow+graupel together to total precipitation.

[andrew-MET]

Can I suggest changing the default from get("harp_params") to getExportedValue("harpIO", "harp_params"). I've made this change throughout harpIO as it enables it to work without having harpIO attached to the global environment.

[pollyaschm]

I changed the default as you recommended.