plot implementation

Only 2D functions are implemented at the moment. Before going for full 3D, I'd like some opinion and comments.

A few things to note:

1. Hoppings and any line related objects cannot have array-like colors (i.e., all lines for the same Scatter object has the same color). Can be corrected by making an list of Scatter objects with different colors... although, not very efficient imho.
2. Site_size can only support pt for now as it seems the easiest to convert between pt and px (native to plotly).
3. I am still not entirely happy with how the intertwining of the matplotlib related functions are used with the color maps. At the moment, I am still at a loss on how to convert it without mpl.
4. And yes, I do apologize for rebasing this before !264 (merged) has been accepted, but I would like to trigger a discussion on the plot implementation before making more changes...