Preprocessor generality

[ledm]

Some preprocessors like zonal_mean and area_average have recently (#825) been expanded to allow more generic operation beyond just a simple mean. This means that while they're called mean or average, they can also do medians, standard deviations, variance, etc.

It would be great to add this functionality to the other spatial and temporal preprocessors:

• average_volume
• seasonal_mean
• time_average

While we're there, we should also change zonal_mean's argument from mean_type to operation to match average_region.

We wouldn't even need to change our recipes if we kept the current preprocessors names like zonal_mean or average_volume as special cases of the more generic functions.

[jvegreg]

#87 is doing this for time related functions

[mattiarighi]

I think this is solved, @ledm ?

[ledm]

Yes!