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