Implemented a way to invert the drift velocity direction inside the ATTPC. Also fixed the padplane-projected hits issue by adding a charge threshold...

[RealAurio]

Small change. Added a geometry parameter on AtDigiPar which determines if the drift direction is inverted or not:

• 0=False => The drift direction is not inverted. In the context of ATTPC, the padplane is at the exit of the detector.
• 1=True => The drift direction is inverted. In the context of ATTPC, the padplane is at the entrance of the detector.

If this parameter is not set in the .par file, the default value is set to 0, so that all previous .par files should still work as expected.

I also added a quick fix for a test simulation from #257 which removes the padplane-projected hits. I thought those had something to do with the inversion of the drift velocity, but they ended up just being produced by the AtPSA when no peak can be found for the pad. Adding a charge threshold of 5 removes those.