Make validation() methods optional and class specific

[dylanhmorris]

Right now, validate() is a mandatory method for subclasses of RandomVariable, but many actual random variables do not have meaningful validation implemented.

We should decide whether most/all can and should have to have meaningful validation. If so, we should implement. If not, perhaps the base class method be a concrete pass that gets overridden / extended by subclass validation methods.

See #275 (comment)_

[dylanhmorris]

We should make a decision on this and move forward. Leaning toward removing validate as a mandatory method, and letting individual RVs define validation or not as appropriate. @damonbayer @sbidari thoughts?

[damonbayer]

Agree with making this optional.

[sbidari]

Agree!