use xray for datastores and netCDF I/O.

[jhamman]

RVIC currently has a lot of homegrown datastores that are trying to replicate the netCDF datamodel. Adopting the xray data structures would dramatically reduce the amount of source code in RVIC. I think a internal refactor would be pretty straightforward and would reduce the overall brittleness of the model.

[bartnijssen]

All for it - but maybe wait till after the RASM meeting