Possibly missing op functions: reduce mean/std/variance, std, variance?

I've noticed that a few ops present in the python docs are missing for reasons I can't determine. Specifically reduce_mean, reduce_std, and reduce_variance. reduce_mean seems like it is just missing the reduce wrapper (it's present in math.mean) but I can't find methods for standard deviation or variance at all. I thought it might be related to #117 so I went looking through the api def protos, and there aren't any standard deviation or variance protos, but there aren't any reduce op protos either so I'm not sure what's up. Regardless of whether this is #117 or not, variance at least would be nice to have.

Start by comparing the Python-documented reduce_mean, reduce_std, and reduce_variance operations with the existing math.mean implementation and the API definition protos referenced in the issue. Check issue #117 for related context; done means the intended operations and their proto or wrapper requirements are resolved and covered by the Java API.