mrtrix: init at 3.0_RC3_latest

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.