ExaTN CMAKE MPI

[DmitryLyakh]

For some reason, ExaTN's CMAKE is still discovering MPI (see below). Didn't we turn that off to make sure the MPI is determined by the user?

-- Found MPI_CXX: /usr/local/mpi/openmpi/git/lib/libmpi.so (found version "3.1")
-- Found MPI_Fortran: /usr/local/mpi/openmpi/git/lib/libmpi_usempif08.so (found version "3.1")
-- Found MPI: TRUE (found version "3.1")
-- Found CUDA: /usr/local/cuda (found version "10.0")
-- MPIRUN: /usr/local/mpi/openmpi/git/bin/mpiexec

[DmitryLyakh]

Also, this causes exatn-config produce an invalid linking line (I built with MPICH, but it show OpenMPI):
-rdynamic -Wl,-rpath,/home/div/.exatn/lib -L /home/div/.exatn/lib -lCppMicroServices -ltalsh -lexatn -lexatn-numerics -lexatn-runtime -lexatn-runtime-graph -ldl -lpthread -lexatensor /usr/local/mpi/openmpi/git/lib/libmpi.so /usr/local/gcc/8.2.0/lib64/libgomp.so /usr/lib/x86_64-linux-gnu/libpthread.so /usr/local/gcc/8.2.0/lib64/libgomp.so /usr/lib/x86_64-linux-gnu/libpthread.so -lgfortran /usr/local/cuda/lib64/libcudart.so /usr/local/cuda/lib64/libcublas.so /usr/local/cuda/lib64/libcudadevrt.a /usr/local/cuda/lib64/libnvToolsExt.so /home/div/intel/mkl/lib/intel64/libmkl_intel_lp64.so /home/div/intel/mkl/lib/intel64/libmkl_gnu_thread.so /home/div/intel/mkl/lib/intel64/libmkl_core.so /home/div/intel/mkl/lib/intel64/libmkl_rt.so -lpthread -lm -ldl /usr/local/mpi/openmpi/git/lib/libmpi.so /usr/local/mpi/openmpi/git/lib/libmpi_usempif08.so /usr/local/mpi/openmpi/git/lib/libmpi_usempi_ignore_tkr.so /usr/local/mpi/openmpi/git/lib/libmpi_mpifh.so /usr/local/mpi/openmpi/git/lib/libmpi.so

[DmitryLyakh]

We need to disable CMAKE automatically discover MPI and make sure the user-chosen MPI library is used throughout, including exatn-config --libs.