The implementation uses extrapolation of strings in the
"Expressions" framework.
There is how:
* $(tech_name) -> substituted by the technology name
* $(tech_dir) -> substituted by the directory the technology file is stored in
* $(tech_file) -> substituted by the absolute path to the tech file
* $(appdata_path) -> substituted by KLayout's home directory (e.g. ~/.klayout)
* $(env('X')) -> substituted by the environment variable $X