Update MrBayes to 3.2.7a and correct MPI variants (#13929)
Update MrBayes to 3.2.7a and correct MPI variants (#13929)

* Update MrBayes and correct MPI variant

* Use tarballs and remove autotools dependencies

* MrBayes: remove ISA variants

iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack