better traits for is_process, use BaseProcess as base of _all_ process types

Environment/IMagneticFieldModel.h

Environment/IMediumModel.h

Framework/Cascade/CMakeLists.txt

Framework/Cascade/Cascade.h

Framework/ProcessSequence/BaseProcess.h

Framework/ProcessSequence/BoundaryCrossingProcess.h

Framework/ProcessSequence/ContinuousProcess.h

Framework/ProcessSequence/DecayProcess.h

Framework/ProcessSequence/InteractionProcess.h

Framework/ProcessSequence/ProcessReturn.h

Framework/ProcessSequence/ProcessSequence.h

Framework/ProcessSequence/SecondariesProcess.h

Framework/ProcessSequence/StackProcess.h

Processes/ObservationPlane/ObservationPlane.cc

Processes/ParticleCut/CMakeLists.txt

Processes/ParticleCut/ParticleCut.cc

Processes/ParticleCut/ParticleCut.h

Processes/TrackWriter/TrackWriter.cc

Processes/TrackWriter/TrackWriter.h

...k/GeometryNodeStackExtension/GeometryNodeStackExtension.h