Create separate app and libraries directories

[astrogeco]

Is your feature request related to a problem? Please describe.
Having libraries in the apps folder can be confusing especially for beginners.

Describe the solution you'd like
New directory called "lib" or "libs" where libraries can "live"

Describe alternatives you've considered
Add documentation justifying why it "makes sense" to keep libraries in the apps directory

[jphickey]

This already exists, there is a "libs" dir (equivalent to apps) as part of the search path already. We just don't use it in the bundle. All we need to do is move e.g. "sample_lib" submodule from apps into libs in the bundle and it should just work.

[astrogeco]

Awesome! I can try that next week