checkInputs used to be added to nativeBuildInputs. Now we have
nativeCheckInputs to do that instead. Doing this treewide change allows
to keep hashes identical to before the introduction of
nativeCheckInputs.
This package consists mostly of command-line tools, but there are also two qt5-based image viewers (`mrview` and `shview`). I have provided an option `withGui = true` to allow users to disable these viewers. They work on NixOS but unlikely to work out-of-the box on non-NixOS Linux (see #62169), but I've refrained from asserting that they shouldn't be built since a user might want to fiddle with runtime GL stuff.
Some programs rely on FSL and/or ANTs backends; I have somewhat heavy-handedly wrapped all binaries to point to ANTs, while FSL is not available in Nix at the moment and has a non-commercial license so would be convenient to disable by default for Hydra builds.
