Fix outputs

I have modified the existing fix langevin implementation to include a stadium type thermostat, where the langevin coefficient varies with distance to a predefined boundary.

I would to validate the implementation by letting the fix give an output of the langevin coefficients, so that i can visualize them in a dump file or plot as a function of distance. Looking at the other fixes, i am little bit confused on how to do this.

Any help is much appreciated.

Best regards