Share filters between templates in ifcodegen

It would be good to share the same filter code and filter.py between different templates when using ifcodegen. There are 2 main use-cases for this in QtAA:

• We create one single way to describe car interfaces, but we want to make different implementations for the backends. In this case, filters are always used to extract the same data from ymls, so putting the same filters.py everywhere gets unmaintainable after a while. Example this and this
• Similarly, when we make tests for API generator functions, the test will have a custom template part to test functionality, but it needs the original filters above. These tests would run best with the original filters.py above.