make drc rule files standalone

[proppy]

Slightly related to #16 and #39, it would be great to decouple each rule files from the global variables defined in main.drc and make them so that they can be executed as a standalone drc script in klayout.

[atorkmabrains]

@proppy It's impossible to do this as the main.drc contains all the base layer definitions and reading procedure. We designed the rule deck structure this way to make it cleaner and easier to run. In the python script, you could run a single table using --table option.