Add additional `**kwargs` for opening a grid file with `ux.open_dataset`

[philipc2]

Related to #385 & #384

Using ux.open_dataset with a separate grid and data file does not allow for passing in arguments in the subsequent xr.open_dataset call that happens in ux.open_grid, restricting the user (see #385)

Add the ability to pass through **grid_kwargs or similar into ux.open_dataset to specify the behavior of xr.open_dataset when opening a grid file