Improve data input-output

[geoglrb]

This PR is the first in an effort to implement better data input and output options for enfolding. Changes are both substantive and organizational in the code base. They include:

• An input-output section of the sidebar in the editing mode been added. Current efforts include adding GraphML inputs for relation data and adding more flexible coordinate systems for images. All such input-output buttons are currently not enabled in this fork, as they aren't ready for use.
• Moved invocation of p5.js from 'instance mode' to 'global mode'. In doing the input-output changes, this appeared to be a good change. As it alters how p5 is invoked throughout the code, it's important to get it out of the data-io branch into the master branch sooner rather than later.

[geoglrb]

FYI, @xrwang, we've made a few changes!

[xrwang]

🎉