Use esa-esdl/gridtools directly

[JanisGailis]

We currently use resampling routines for coregistration from gridtools by copying the code to ops. We should use the library directly by importing it because:

• We can, it's MIT licensed
• Not doing so is a huge anti-pattern resulting in code divergence, not contributing potential improvements upstream, as well as not leveraging potential improvements made upstream
• We also want to use it for gap-filling #81