Add a method for joint entropy

[Pierre Denis]

Since Lea provides several methods related to information theory (information, mutual information, entropy), it would be interesting to have a convenience method jointEntropy that calculates the jointEntropy of a set of Lea instances args. Given the current Lea methods, the value is calculated with

Lea.cprod(*args).entropy

[Pierre Denis]

Add condEntropy and jointEntropy methods (refs #37, #38)

→ <<cset 8e684ae66394>>

[Pierre Denis]

Add condEntropy and jointEntropy methods (refs #37, #38)

→ <<cset 8c295dcd8efe>>