Cross Validation needs to maintain chunking scheme

[kjhall01]

every slice in the cross-validation process should return the same chunking scheme as the initial arguments, X & Y. Currently, It sometimes re-chunks across the S dimension (likely a by-product of xr.concat in the function definition).

Either have XCast accomodate multiple chunks along S/M (ideal) , or have cross validator maintain number of chunks along S/M.

[kjhall01]

Actually, XCast does accomodate multiple chunks along S/M. The problem here is xc.RankedTerciles, which uses xr.quantile which in turn uses dask apply_gu_func, which does NOT accommodate multiple S/M chunks.