[enhancement] MRCat interface for mrtrix3's mrcat function

[GalKepler]

Summary

Would be nice to have an interface for mrtrix3's mrcat function as other alternatives (fslmerge etc.) can't handle the mif format.

Expected behavior

import nipype.interfaces.mrtrix3 as mrt
mrcat = mrt.MRCat()
mrcat.inputs.in_files = ['image1.mif','image2.mif']
mrcat.cmdline
mrcat image1.mif image2.mif concatenated.mif
mrcat.run()

Platform details:

{'commit_hash': 'aa64ccdfb',
 'commit_source': 'repository',
 'networkx_version': '2.6.3',
 'nibabel_version': '3.2.1',
 'nipype_version': '1.7.1-dev+gaa64ccdfb',
 'numpy_version': '1.21.3',
 'pkg_path': '/home/groot/Projects/nipype/nipype',
 'scipy_version': '1.7.1',
 'sys_executable': '/home/groot/Projects/dwiprep/venv/bin/python',
 'sys_platform': 'linux',
 'sys_version': '3.9.5 (default, May 19 2021, 11:32:47) \n[GCC 9.3.0]',
 'traits_version': '6.3.1'}