Add helper accessor to return final time slice

[mikekryjak]

This returns the final time slice of a Dataset or DataArray whether the data contains multiple time slices or not. This is helpful when working with multiple simulations, some of which may have only been run for one timestep for developer reasons.

Here is how this can be done with built-in Xarray functionality right now - let's say we are reading in a 1D model, want to get the final timeslice whether the dataset contains time or not, and then snip off guard cells:

ds = ds.isel(t=-1, missing_dims = "ignore")
ds = ds.isel(y = slice(2,-2))

With this PR we can now do:

ds = ds.bout.final_time().isel(y = slice(2,-2))