Inconsistent `.mutual_information` method

[grfrederic]

In our samplers .mutual_information() is a callable method, in tasks it is a @property. It would be nice for them to be consistent.

[pawel-czyz]

@grfrederic The reason for this is the following:

• ISampler may need additional arguments to calculate mutual information (as we do with BMMs, which collect MC samples. Here one can potentially specify the required number of samples. I also recall that we used it in some discrete-continuous estimators where MI is analytically known as an infinite series and we truncate it after a given number of terms).
• Task is supposed to already know MI. I.e., for ISampler .mutual_information(k) may have complexity $\mathcal O(f(k))$ and for Task querying .mutual_information property is supposed to be $\mathcal O(1)$.
• How about adding .mutual_information() to Task for consistency?