Add func argument to compute_MVBS

[ctuguinay]

The addition of the func argument will enable users to specify the aggregation function used by the underlying flox.xarray.xarray_reduce. This func parameter determines how the aggregation is performed on the Sv data array. By default, this function is set to nanmean, but users can opt to use other aggregate functions like mean in the case where they want to NaN any MVBS bin that 'contains' NaN values in the original Sv grid.

One can find all available options for func listed here: https://flox.readthedocs.io/en/latest/generated/flox.xarray.xarray_reduce.html.

[leewujung]

We close this now with #1269 merged. Also that since compute_MVBS is supposed to be a mean or nanmean function, we don't need to include the flexibility of allowing different func. But the intermediate function under the hood does allow that and we'll likely use them for other processing functions later.