Code tree reorganization

[apjanke]

Implement code organization updates towards the goal of fewer files per folder (package folders or otherwise). For instance:

1. In Utils package: consider separating out the pure coding utilities from the domain-oriented npix utilities
2. In root package: consider separation of some/all functions from the classes, perhaps via an additional sub-package
3. In Neuropixels package: consider separation of related function such as for file reading/writing/exporting (e.g. readNPY & writeNPY) & consider whether some functions (e.g. runKilosort2) can become public methods of classes defined in the package