MRG Refactoring the axes module (part II)

This is part of the refactoring process. This PR is still work in progress.

 All plots methods should be moved to their own modules. 

In this PR, I've started the refactoring by moving the axhline to its own module, lines, which should contain lines and spans
plotting function.

What is left to do before merging:
- [x] Study the possibility of using mixins instead of manually updating the Axes class