Clean up radon and iradon to improve readability

The cleanup has two primary goals:

- [x] Use `util.pad` to perform array padding. This depends on #634 being merged first (the functionality is included in    this PR in commit 4395872 to make tests pass; this commit will be dropped in a future rebase).
- [x] Use the interpolation functions in `_shared.interpolation` to perform nearest and linear interpolation.

Please do not merge until both of the items listed above are complete.