Ensure that whenever we add information to obs or var that the user is informed

[Zethson]

Whenever the AnnData object is modified the user should get informed. Scanpy, cellrank and others use the logger to inform the user in such cases. We should also create our own logger instance and add logging info whenever the AnnData gets modified with a corresponding message.

Investigate cellrank for an example of how to create & structure such a logger. Cellrank:

        logg.info(
            f"Adding `adata.obs[{key!r}]`\n       `adata.obs[{Key.uns.colors(key)!r}]`\n",
            time=time,
        )

https://github.com/theislab/cellrank/blob/master/cellrank/logging/_logging.py