Re-organize the code for modularity

[shz9]

Currently, the GWASDataLoader class incorporates many functionalities that make it brittle and harder to work with. We would like to move some of those functionalities into their own classes/objects.

Here are a number of ideas that we can implement:

• [x] Move summary statistics to their own class SumStats
• [x] Move LD functionalities to their own class LD
• [x] Move annotations to their own class 'Annotations`
• [x] (Need discussion) Move genotypes to their own class Genotype

In this sort of setup, the GWASDataLoader object would be an interface to harmonize these data classes, instead of performing many different, unrelated functions.

[dorukcakmakci]

While these changes make code more modular, development can get messy quickly. How about we write a test for GWASSimulator class to ensure it preserves its functionality before tackling this issue?

[shz9]

Most of this refactoring was done in magenpy==0.0.2.