Add AnnotatedPhenotypeSimulator

[shz9]

The current PhenotypeSimulator class has some functionality for simulating with annotations. We need to remove those functionalities and embed them in a new class AnnotatedPhenotypeSimulator that inherits from PhenotypeSimulator and incorporates annotations in simulate_mixture_assignments and simulate_betas.

Tasks:

• Remove annotation_weights property (as well as other related methods) from GWASimulator.
• Create AnnotatedPhenotypeSimulator class
• In AnnotatedPhenotypeSimulator, overwrite the simulate_mixture_assignment method to incorporate annotations.
• In AnnotatedPhenotypeSimulator, overwrite the simulate_betas method to incorporate annotations.

Additionally, we need to figure out two things:

• How to simulate betas according to user-defined enrichments for each (or some) annotation classes.