Add Modelica installation for Linux to Install Guide

[kzscisoft]

Add the installation of OMC to the install guide making clear note that all of OpenModelica is not required, only:

echo "deb https://build.openmodelica.org/apt `lsb_release -cs` stable" | sudo tee /etc/apt/sources.list.d/openmodelica.list
wget -q http://build.openmodelica.org/apt/openmodelica.asc -O- | sudo apt-key add - 
sudo apt update
sudo apt install -y omlib-modelica-3.2.3 omc