mrtrix3: new package (#5568)
mrtrix3: new package (#5568)

* mrtrix3: new package

* specifying conflict dependency ve
LLNL/spack,lgarren/spack,iulian787/spack,mfherbst/spack,lgarren/spack,krafczyk/spack,iulian787/spack,skosukhin/spack,LLNL/spack,matthiasdiener/spack,lgarren/spack,skosukhin/spack,iulian787/spack,iulian787/spack,skosukhin/spack,krafczyk/spack,mfherbst/spack,krafczyk/spack,mfherbst/spack,EmreAtes/spack,LLNL/spack,tmerrick1/spack,krafczyk/spack,EmreAtes/spack,tmerrick1/spack,lgarren/spack,matthiasdiener/spack,skosukhin/spack,matthiasdiener/spack,tmerrick1/spack,EmreAtes/spack,tmerrick1/spack,matthiasdiener/spack,tmerrick1/spack,matthiasdiener/spack,EmreAtes/spack,LLNL/spack,skosukhin/spack,mfherbst/spack,EmreAtes/spack,lgarren/spack,iulian787/spack,krafczyk/spack,LLNL/spack,mfherbst/spack