Allow a centralised PDK installation of KLayout and the IHP PDK modules

[hpretl]

In some situations (like in our IIC-OSIC-TOOLS image, see https://github.com/iic-jku/IIC-OSIC-TOOLS) the PDK sits in a read-only location, and even the user home directory is almost read-only.

How should one setup KLayout and respective environment variables properly for this to work? E.g., accessing the right .lyt and .lyp files, and especially finding and binding all the Python stuff? I am not sure that the current setup (if I look into the .lyt for example) supports such a usage model, and if it does, how to set up the user environment accordingly.