API: refactoring `AbstractGenerator` interfaces

[Desiment]

Currently, AbstractGenerator provides two methods for creating samples:

• canonical_generate(mixture, size)
• classical_generate(mixture, size)
  Which both do the same job, but one is suitable when mixture defined using classical form, and one is suitable when mixture defined using reduced (a.k.a. canonical) form.

This behavior should be changed to the following. Subclasses of AbstractGenerator should have one method generate(mixture, size) which automatically understands in which form mixture defined, and converts reduced form to classical if necessary.