refactor pixel selection methods to allow custom classes

[vincentsarago]

closes #1 #2

⚠️ This is a major update and will break the previous release

This PR adds an abstract base class for pixel_selection method, allowing custom classes. The base class is relatively simple with 3 methods/classes

• is_done: a property to check if the process should stop
• data: translate internal numpy masked array to rio-tiler like tile, mask tuple
• feed: add data to tile and compute (main feature)

This work is mostly inspired by https://github.com/mapbox/watchbot-progress-py

I'd love to get feedback on

• the python style and logic (this is the first time I'm using abstract classes)
• the words (e.g class names, variable names...)

Note: This PR removes the last methods because it's was making things more complicated and the logic should (IMO) be moved to the top level app.

[vincentsarago]

here is a gist showing the new usage and how to create simple custom methods https://gist.github.com/vincentsarago/5c71f4a91115277dd73f92f7577faad2