Output variable statistics

[gutmann]

The 2.0 implementation of output variables uses objects, and could be extended to permit output variables to "compute" themselves prior to output, e.g. winds could rotate to east-west/N-S coordinates, accumulated_precip could add a bucket, time averaging or min/max statistics could be added.

It is not clear how some of this would work for variables that are output to multiple files (e.g. restart and standard output files), would separate variables need to be instantiated in each output object?