Move the methods related to the instrument state to their proper modules (better coherence)

• The diffractometer-related methods (state, detectorStates and sampleStates) are moved from IDataReader to Diffractometer class.
• Computations of the instrument state is performed in the InstrumentState class.
• InstrumentStateList class is retired to avoid possible undefined behaviour. Note that inheriting from STL containers is unsafe as they do not have a virtual destructor; see eg., https://stackoverflow.com/a/7110262.
  InstrumentStateList class was simply a vector of InstrumentStates with an extra interpolation method which is now moved to InterpolatedState class.