Add TRX support to tckconvert

[mattcieslak]

This adds picks up where #3262 left off, adding TRX support to tckconvert, updates the documentation and adds tests. It depends on MRtrix3/test_data#6 to pass the tests.

I haven't added trx-cpp to any of the builds - I was hoping for some input on how would be most convenient to do so.

Thanks in advance!

[daljit46]

Currently, we handle dependencies through CMake's FetchContent with optional flags to use a system-provided version of the given library. I think the same approach should work fine with trx-cpp. One issue would be that trx-cpp depends on libzip and expects that to be installed on the user's system. So I see two options here: either mandate the installation of libzip for building the project (as we do for Qt) or we also fetch libzip via FetchContent when trx-cpp support is required. @jdtournier @Lestropie any thoughts?

@MRtrix3/mrtrix3-devs as an aside, I think this issue highlights some of the limitations of using FetchContent for dependency handling, as dealing with transitive dependencies is not straightforward and we may want to consider adopting a package manager like vcpkg.

[jdtournier]

I think we should keep it as simple as possible for now, and document the expectations sufficiently for other developers. As long as regular users can use the binaries we produce, they won't care either way. It's a relatively small subset of people who might benefit from transitioning to a more complex package manager, and few of them will be particularly upset if they need to install other packages manually. Realistically, I don't think it's worth investing much time in finessing the developer experience...

The bigger issue is whether we need to bundle these additional dependencies in the binary packages (which I assume we should?).

[Lestropie]

I'm not perturbed about specifically inheriting a libzip dependency given I'd like to support the .npz format (#2682) that I think would itself require libzip. Not to say that transitive dependency isn't a problem, just that this particular one doesn't scream problematic to me. It's pretty common as a source compilation dependency and stable as an API, and the runtime dependency will be ubiquitous, so I'd just mandate it at system level.

[daljit46]

@mattcieslak Given the comments above, I think adding something along the lines of:

if(MRTRIX_USE_SYSTEM_TRXCPP)
    find_package(trx-cpp CONFIG REQUIRED)
else()
    set(trxcpp_url "https://github.com/tee-ar-ex/trx-cpp")
    FetchContent_Declare(
        trx-cpp
        DOWNLOAD_EXTRACT_TIMESTAMP ON
        GIT_REPOSITORY ${trxcpp_url}
        GIT_TAG ea9ebcbf4b08e047e6493eb8077f789cdfac36b7
    )
    FetchContent_MakeAvailable(trx-cpp)
endif()

in cmake/Dependencies.cmake should do the trick. We'll also need to update our build instructions and CI workflows for the installation of libzip.