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.

Comienza comparando las operaciones reduce_mean, reduce_std y reduce_variance documentadas en Python con la implementación existente de math.mean y los protos de definición de la API referenciados en la issue. Consulta la issue #117 para ver el contexto relacionado; se considera terminado cuando las operaciones previstas y sus requisitos de proto o wrapper estén resueltos y cubiertos por la Java API.