sibling of #380

This PR attemps to fix some of (or all) the typing issues found by running mypy histolab.

Unfortunately some of these issues are hard to fix without context and without making a design decision. I've left a couple of TODOs in the code for that.

The biggest issue is having class Filter(Protocol) that has a call method that accepts either an image or a ndarray, unfortunaly mypy doesn't allow to change the argument type to a more specific one (see https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides). Maybe we should have two types of Filters, one for images and one for ndarrays.

Also, I'm not 100% convinced Protocol is useful here (and in the Tiler class), what's the reasoning behind using Protocol and not abc.ABC?