Figure builder class feature.

Hi,

Thank you for this wonderful tool!

I recently forked the project (https://github.com/gucky92/seaborn.git), and have been playing around a bit. I have added two main features to my fork and wanted to know if there is general interest or consideration to implement something similar.

First, I made some changes to the Grid classes to allow passing a figure and subplot_spec argument in the init. Thus, a Grid figure can be part of a larger figure using gridspec.GridSpecFromSubplotSpec. This also allowed me to build a Figure builder class that can include individual subplots and Grid plots. This Figure builder class also allows for adding panel letters to each axis.

Second, I added a scalebar function that uses some of the seaborn functionality to convert the x-axis and/or y-axis to a scalebar. I added a method to the Grid class to convert the axes internally to scalebars.

Here is a MyBinder with quick examples of the two features:

Please let me know what you think!