Complete clean up of the documentation

[mrossinek]

Extracting the tracking of documentation cleanup from #37 into a separate issue:

• [x] quantum_info module (#51)
• [x] library module (#40)
• [ ] sampler module
• [ ] post_processor module
• [x] utilities file (already good)

As part of this undertaking:

• [ ] re-evaluate the private/public attribute distinction on all classes