Covariance method only for Alea instances

[Pierre Denis]

The cov method is only available on Alea instances, i.e. independent random variables. This makes its usage quite limited:

a.cov(b)

• raises exception if a is not an Alea instance,
• = a.var if a is b,
• = 0 if a is not b and b does not refer to a,
• = a non-trivial covariance in other cases.

a.cov(b) should work on any Lea instances.

[Pierre Denis]

Refactor covariance-related methods + add UT (refs #57)

→ <<cset 3125c695eeeb>>