Documentation: Fixes for Spack build issues when using core RC packages

[yantosca]

Hi all,

During my Spack builds I encountered a couple of issues that I solved by editing the relevant packages.py files. Please see these links for details:

• Manually force cdo to depend on mpi and netcdf-fortran
• Error: Could not find perl in /usr/bin/bin

I will document fixes for other issues at: https://github.com/Harvard-ACMG/spack-env/wiki.