Adding a Flask backend

[KDW1]

Added an alternative setting for the PathView application so that you can either use Pyodide web assembly or send client-side fetch requests to a Flask web server that has execution, evaluation, and traceback routes (the initialization code of the server borrows from the init() function found in worker.ts.

Major Changes:

• Added a backendPreferenceStore which is nearly identical in form to themeStore since it switches between two binary values. It is binded to 'Q' (arbitrarily so), and toggling it calls switchBackend(preference : BackendPreference) from backend.ts
• The backendPreferenceStore is accessed from backend.ts and passed in as an event parameter sent along with the "init" message posted to the worker. This sets the backendPreference for said worker
• Every function within worker.ts that previously called await pyodide.runPythonAsync(...) now has a fetch request counterpart, such that the Flask backend route is shallow copy of what pyodide was doing

Current issues:

• It seems like we're running multiple excess requests due to an error Scope: 'NoneType' object is not iterable which comes from the generateStreamingStartCode() function in pythonHelpers.ts, specifically the find_scopes() funciton

[KDW1]

TODOS

• Review the requirements for handleRun() in the main index page for the SvelteKit application, in terms of whether simulation is running or not, etc...
• Fix the issue where the Flask version doesn't interact with the console's stdout and stderror
• Fix the Scope: 'NoneType' object is not iterable error, which is most likely connected to the excess api calls!
• Finish pull request description