Add helper functions for weighted averages

[JoyMonteiro]

In climate science, certain weighted means (pressure weighted mean in the vertical, area averaged
mean in the horizontal) are fairly common. We should add some utility functions to perform these
operations so that users don't run into the problems like mcgibbon/sympl#45.

[mcgibbon]

I don't really see the relation to the referenced issue, but yes, having weighted-aware mean utility functions that take advantage of the pressure/lat/lon coordinates to determine weighting would be useful. In general, model-aware (of the assumptions in CliMT) interpolation functions would be useful.