Reduced use of raw pointers.

Reduced use of raw pointers.

- Replaced pointers with references in some member variables and return
  values where the value was never NULL.
- Refactored AnalysisDataHandle to be able to pass them around by value,
  all pointer management happens internally.
- Also improved TrajectoryAnalysisModuleData::dataHandle() to not rely
  on string values (error-prone) when searching for registered data
  objects.

Conflicts:

	src/gromacs/analysisdata/analysisdata-impl.h
	src/gromacs/analysisdata/analysisdata.cpp

Change-Id: I43caade060ad2d4cd991a6bb5541462d9ac96760