initial set of raster extent functions

[elbeejay]

aim is to add some functionality to clip a list of rasters by a common extent (or expand them as necessary to a common extent).

still a lot of work to do on this PR, including but not limited to:

• testing that it works
• being more flexible with respect to the "origin" of the raster
• functionality to "expand" smaller rasters to common extents of larger ones, inserting "nodata" values as appropriate

[jonschwenk]

Looks nice!

You also might want to add a check that the rasters have the same CRS? I think that can get a little hairy if you rely just on gdal's .GetProjection() since it returns a string that may be more/less verbose for the same CRS. I have some functions that try to extract an EPSG code from a gdal projection string but they felt kinda hacky--I would be surprised if there weren't some packages or more streamlined ways to do this.

You should probably also be clear about the fact that clipping rasters to common extents doesn't ensure that their extents align perfectly since they could potentially differ in resolution that lead to sub-pixel misalignments at the borders. You could address that by adding some resampling code, but I think that's more complicated than you want to go for this. Most use cases are likely rasters from the same source, so they'll have the same resolution and CRS so this is moot.

[elbeejay]

Thanks for the suggestions @jonschwenk. A version of what I'm aiming to do exists within the gdal_calc.py script, so I plan to dig through exactly how they accomplish combining rasters of different extents/projections to get some inspiration and see what aspects of that approach I want to keep, adapt, or alter for this use-case.