experiment.h: Experiment class member functions return by reference

[yash4karan]

The following member functions have been modified to return by reference:

const Goniometer& goniometer() const;
const BeamType& beam() const;
Scan& scan();
const Detector& detector() const;
const Crystal& crystal() const;

Only scan() returns by non-const reference for now, as the underlying scan_ member variable is required to be modifiable by the prediction algorithm.