Let Shower detector calculate track projections for all tracks, not just first

This was already done for Scintillator and Cherenkov detectors, but had
apparently been overlooked for the Shower.

Also, delete trailing whitespace.

Closes #44
Cleanup of some detector code (Scintillator, Cherenkov, Shower)

- Move THaScintillator::GetNTracks() into implementation to avoid
  having to include TClonesArray.h in the header
- Drop obsolete member variables tan_angle etc. These have been
  replaced with fXax, fYax, fZax in THaSpectrometerDetector
- Remove unused member variables fTRX & fTRY from THaCherenkov.h
- Delete trailing whitespace
Move track projection handling from detectors to THaNonTrackingDetector

This avoids extensive code duplication and duplicate member variables
between the non-tracking detector classes.

Also, fix GetNTracks() function, which previously always returned the
number of reconstructed tracks. It is more useful to have it return
the number of tracks found to be passing through this detector.