generalised interface for xyz raster files

Generalised the interface for xyz-raster-files. Not all pixels in a given rectangle of coordinates must be present and they need not be in the correct order.

I originally wrote this using a simple stream and later optimised it to use std::optional for coordinates and std::min/std::max for getting limits. Somehow reading a file now takes twice as long as with the straightforward approach. Is there anything obviously slowing down the process? (and I'm not talking about traversing the file twice, I did that in my other approach as well)

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behaviour change was documented?