Convenience functions RandomVariable.var and RandomVariable.std

[nathanaelbosch]

If we want to access the variance and standard deviation often, it might be useful to have those convenience functions. The naming could also be cov and std, which might be more in line with our current style.

EDIT: I just saw that Distribution has those functions. Then I guess my question is: Would it make sense to have those for RVs also? If not, what is the reasoning behind that?

[JonathanWenger]

Absolutely. I believe these may have even existed at some point and gotten refactored. I would suggest to have cov var and std such that cov returns the covariance, var its diagonal and std the square root of var. For univariate rvs cov and var return the same.

[marvinpfoertner]

This issue should resolve itself if we decide to unify random variables and distributions, right? Relates to #131.