How to handle future extensions

[n-garc]

One of the goals of the current improvement roadmap is to enable easy future expansion of the pipeline in a 'modular' way - that is, we want to allow users to 1) select the modules they want to run, and 2) write additional modules that support additional functionality (e.g., new stimuli decoding, etc.). There are a few ways to handle this in terms of where these files live, depending on lab preference. The most straightforward is just to add additional .py files to an extensions folder inside the data-processing folder. This might be annoying and there are ways to make the UX nicer.