USE_SCREENING not implemented for pynucastro networks

[zingale]

We have a USE_SCREENING parameter. If it is set to FALSE, then screening is compiled out (via the SCREENING preprocessor variable).
We don't implement this for pynucastro nets. It's not clear it really is needed though, since we can always set the screening to null.

However, at the moment, the pynucastro nets don't compile if we disable screening with this build flag.