Make sure GLAC can run on modern hardware(Ubuntu 20.04, latest MacOS): update the installation guide on how to make it run

[hmvege]

The installation of GLAC is outdated, and may no longer work on some systems. For instance, there are issues related to running MPI on the latest Mac OS.

• Update the installation guide for Mac OS
• Update the installation guide for Linux(Ubuntu, specifically)

Verify that the program works in both cases.