Review docstrings / fill the gaps

[neuroBazzi]

@phildong Hi Phil, when you have a second can you please take a look at the docStrings Initialization/Utilities/Visualization?

In every file there a few inputs/outputs that myself and Zoran could no understand the data type of. For ease of reference I will add your email to those comments.

[phildong]

• [x] cnmf
• [x] cross_registration
• [x] initialization
• [x] motion_correction
• [x] preprocessing
• [x] utilities
• [x] visualization