[MAINT] Fix image processing module.

Add unit tests for each submodules and fix everywhere if necessary:
- [x] Deriche
- [x] Interpolation
- [ ] Linear filtering.
- [ ] Differential
- and so on...

Let's not do everything at once and break down this boring maintenance task into smaller PRs.
Modifications in these files are partly obsolete since there were done a long time ago. So check these very thoroughly.