Intensity and Amplitude design rework and log likelihood estimator
tests:
- Huge rework for Intensity and Amplitude related classes (Closes ComPWA#130)
  The instrinsic normalization feature was removed, as well as the
  PartialAmplitude and Resonance class. Different Amplitude classes
  follow the decorator pattern and can be combined! Because the
  normalization feature was removed also the MinLogLH class was
  reworked completely.
- The classes Intensity, Amplitude, Kinematics are now pure interface
  classes.
- Introduced a bunch of builder functions in the IntensityBuilderXML
  namespace which are now responsible for creating the intensities,
  amplitudes, and kinematics objects from xml files. This removes the
  interdependencies of concrete instances of these object. Also the new
  builder functions can now more easily create arbitrarily complicated
  intensities from the xml file (Closes ComPWA#141).
- The Estimator interface was improved.
- FunctionTree feature for Estimators was decoupled to improve
  modularity. Now there exists a FunctionTreeEstimator wrapper class.
- Improved the Event and DataPoint data structures. They are now simple
  data structures, which are stored in std::vector containers.
- Started to remove unneccesary shared_ptr usage, which can lead to bad
  code design.
- General code improvements, for code that was related to the rework
  above.
- Introduced a test cases using the log likelihood estimator
  (MinLogLH). One test performs verification of error estimations also
  on events with weights (Closes ComPWA#44).
- Changed the hierarchy of the logging levels to TRACE, DEBUG, INFO,
  WARNING, ERROR, FATAL (default: INFO)