Install Spack + Quantum ESPRESSO on MacOS

[nguyen-group]

1) Install Brew (https://brew.sh/):

$ /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"

2) Install Spack from Brew:

$ brew install spack

3) Find the compiler in spack:

$ spack compiler find

4) Checklist of compilers

$ spack compilers

5) If there is no GCC, install GCC

$ spack install gcc

5) Install Quantum ESPRESSO v7.2 (v7.3.1 might not work yet)

$ spack install  quantum-espresso@7.2