lazy nc data saving

adaptions to netcdf save to always save by slicing 2D data elements from the payload  and streaming these to the netcdf file


I think we want to avoid data operations in the netcdf save and keep biggus in between the save code and the data payload.  This is the larger overhead, imo.

Biggus always returns the type of array (maskedarray|ndarray) you ask for; i'm not sure it provides the idea of 'native type' or something similar to support a decision gate in the code path.