OpenMPI installation - make it more explicit?

[geerlingguy]

Currently the README specifies:

An installation guide for openmpi can be found inside the tarball

But since a specific version is called out, and replication requires more specifics, I would suggest adding the steps to download and compile it:

popd /opt
wget https://download.open-mpi.org/release/open-mpi/v4.1/openmpi-4.1.4.tar.gz
tar -xvf openmpi-4.1.4.tar.gz
cd openmpi-4.1.4
sudo apt install -y gfortran
./configure
make all install
export PATH=/usr/local/bin:$PATH
export LD_LIBRARY_PATH=/usr/local/lib:$LD_LIBRARY_PATH

This installs OpenMPI in the default paths, and explicitly sets them in the exports, making the rest of the setup process straightforward assuming a default 20.04 install (I had to re-reference my own notes in #3 to do this again).