Shared custom modules (e.g. colormaps, styles, maths)

[ptuemmler]

It would be cool to have the ability to use the same modules/functions in different apps without having to copy/paste them into every app individually. This would be particularly useful for a consistent colorscheme and style and also for maths stuff that might appear in different apps (rk4 solver or some such).

An idea how this could be done:

1. Create a common_functions folder containing different python scripts that can be used in all apps.
2. Allow an app_functions.py file in the source directory of a specific app, which can be used in the sub-apps to a given topic
3. Copy the actually used modules/functions into the app directory created during the mkdocs process such that they are available there.
4. Change the import logic during the mkdocs process such that they are imported correctly from the newly copied files.