Adding Scanpy's logging/settings & Scanpy functions

[Zethson]

Open discussion whether or not we should reimplement Scanpy algorithms or add it as a dependency.

• [x] Add a custom settings thing analogously to Scanpy with our own design (path to already downloaded datasets etc)
• [x] Add all Scanpy functions that we want and need with custom docstrings and wherever possible without single-cell specific parameters

[Zethson]

Regarding logging & settings:

1. It would be easiest to solely use Scanpy's logging and settings & to integrate it into our code
2. A nicer experience would be to write a new logger & settings based on Rich. In this case it likely would have to either monkeypatch Scanpy's logging & settings or the new custom logger would as a side effect also control Scanpy's equivalents.
3. In any case, our own download & caching etc must respect the logging & settings

Relevant links:

• https://github.com/theislab/scanpy/blob/master/scanpy/_settings.py
• https://github.com/theislab/squidpy/blob/6bc70401cb34e514fd736517d41889765be6b5eb/squidpy/_utils.py