Create a new sub-folder "plot"

[michaelchin]

The code base related to plotting is getting large. I am considering the possibility of adding a new sub-folder "plot" and organise all the plotting code in the new sub-module.

1. Move plot.py into sub-folder "plot" and rename the file properly.
2. Move all files in folder "mapping" to the new folder "plot".
3. Move utils/plot_utils.py to the new folder "plot" and rename the file properly.
4. use re-export to keep backward compatibilty.

@jcannon-gplates

This is an idea popping into my head today. I would like to hear your thoughts. The sole purpose of this effort is just to organise code in a better way.

[michaelchin]

And also the file grids.py is also quite large (>3000 lines). Maybe we should break grids.py into smaller files as well.

[jcannon-gplates]

That all sounds good to me 👍