Improve readability of active/inactive condition

This, as is, is rather minor but it's on the way towards improving the time-stepping.

I have replaced all explicit checks that a particle or cell is active by calls to inlined functions. No change in functionality thus far.

The plan is to slowly introduce a better time-stepping mechanism where the particles carry less information (lower footprint) and make the whole time-step limiter problem easier. With this merge request in then I only need to make the changes in one single location (file active.h) and not everywhere.