Change build system to CMake

[daljit46]

This PR supersedes #2620 (which you should check out for the motivations for this proposal). It adds support for building MRtrix3 using CMake.
A major change is that we now build all the code (except for the commands) as a shared library that links to each executable.
As far as I can tell, this is branch should be feature complete and shouldn't miss any major functionalities compared to its base branch. Instructions to build and test can be found here.

What's missing?

• Tweaking of CI checks to match the settings of our current workflows.
• Addition of secondary checks in the CI to run Pylint and update copyright headers.
• Handling of the files inside the share directory.
• Removal of possible redundant files that are closely tied to the old build system.
• Other minor details in the CMake scripts.
• Update documentation.

All of the above would need to be discussed before this is ready for merging.
@MRtrix3/mrtrix3-devs if there is anything needs to be added or changed or something I missed, please let me know.

[daljit46]

@MRtrix3/mrtrix3-devs For the full documentation of the new build system, shall I just create a new document that details how to build the project? Or is it better if I wait until we actually start using the branch in practice to obtain more feedback?
The cmake-instructions file provides the basics but a more thorough explanation is probably warranted.

Furthermore, we currently have two documents (one in the user documentation directory and the other in the developer documentation), both of which explain the build process with significant overlap. I propose that the we should get rid of the any documentation regarding building MRtrix3 from the user documentation and instead provide a single page where the CMake build process is documented for developers.