Improving entropy calculations

[matteobecchi]

Requested Reviewers: @andrewtarzia @SimoneMartino98
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Following the entropic chaos of the last days, I have

• added an argument to the functions for the entropy calculations to set the unit of measure of the output
• added a function to compute the entropy of multivariate datasets using KL estimator
• added a recipe that explains how to compute the entropy of different variables

Everything should be done so that old code still works correclty.

En passant, I also have solved a couple of minor bugs, both in the entropy calculations and in the recipes (the download link wasn't working).

I think I want to improve/clarify the "information gain" stuff, then we should be OK.

[matteobecchi]

Ok, if everything runs smoothly, I am done here!