Up- or downsample probabilities between zoom levels

Right now rs masks is able to combine multiple slippy map directories with probabilities. The idea is that we can have multiple models and this feature allows us for model ensembles.

We should add functionality to upsample or downsample slippy map directories with probabilities based on zoom levels.

The use-case is as follows:

• we train models to detect roads on zoom level 19, very close up with details
• we train models to detect roads on zoom level 16, further away high-level view
• we predict and get two slippy maps probs/19/x/y.png, probs/16/x/y.png
• we want to combine these predictions into masks on a specific zoom level, say z18
• we need to downsample the z19 predictions and upsample the z16 predictions

We can either extend rs masks to upsample or downsample. Or we add a new tool which can transform a slippy map directory on zoom level z0 into a slippy map directory on zoom level z1, with z0 != z1.

Implementation notes:

• mercantile has functionality for children and parent tiles
• pillow has functionality for up- and downsampling

Start with the existing rs masks functionality that combines slippy map directories with probabilities. Read mercantile's child and parent tile support and Pillow's upsampling and downsampling capabilities, then trace the z19 and z16 prediction flow toward a target such as z18. Done means predictions from differing zoom levels can be transformed and combined into masks at the requested zoom.