Remove Geometry objects and make them part of the respective artists

[v0dro]

As per #1 , we are in the process of separating the backend from the Artist layers. This makes the design of the code very object oriented and nicely encapsulates data within many varied elements for any given plot.

Therefore, there is now no need of having various Geometry objects for holding data.

This refactoring will involve putting things from one or many Geometry objects to the Aritist where it is actually useful with the aim of removing the Geometry object from the code base altogether.

This issue can be broken down into several sub-issues:

• Remove Bar::Geometry object.
• Remove Line::Geometry object.
• Remove Scatter::Geometry object.
• Remove Artist::Geometry object.

Each of the above tasks should be a separate PR/commit.