Defining functions in folder_defs and preprocess_traces

[mariacozan]

Added descriptions to function define_directories and create_processing_ops in folder_defs and to function zero_signal in preprocess_traces.

[schroederlab]

Looking good. Here are few comments:

• define_directories: (1) function creates strings of important directory paths (the return variables don't contain the directories); (2) Nice and detailed description of what the directories need to contain! Would be great if we had an example csv file on github, as small details (like the format of dates etc) matter a lot. (3) The description of s2pDir is somewhat confusing at the moment as (I think) the described files do not need to be directly in the specified folder but in (very specific) subfolders. Please specify the assumed folder structure (something like \s2pDir[animal][date]) and which folder contains which files. (4) Please specify what metadata means and which folders exactly are expected to be in metadataDir.
• I noticed that many folder definitions are strings using "\". This won't work on all operating systems (e.g. Mac), so needs to be exchanged. Liad knows how to. Please write an issue for him, wherever you see this being used.
• Although not super important it would be nice to stick to the same style everywhere. In my last comments I said to start function descriptions with an active verb, so not "This function creates..." and not "Creating..." but "Creates...".
• If users want to change preprocessing settings, they need to change "pops" in create_processing_ops? If yes, please make this very clear to the user.
• If people continue to make changes to file folder_defs.py (to change folders and to change settings), will this file be set to "ignore" on github? (ask Liad or write issue)