Now using the additive error measure (and distance measure) instead of the multiplicative one. New sub-namespaces. Miscellaneous other changes.

- Changed the Point class so that it uses the additive error measure.
  + Moved Point::dominates()'s code to Point::dominatesMultiplicative().
  + Added a Point::dominatesAdditive() method.
  + Now Point::dominates() simply calls Point::dominatesAdditive().
- Changed the Point class so that it uses the additive distance measure.
  + Added a Point::euclideanDistance() method. (in case the user needs it)
  + Added a Point::additiveDistance() method.
  + Added a Point::distance() method, which simply calls
    Point::additiveDistance().
  + Changed every external call of Point::ratioDistance() to
    Point::distance().
- Changed the Facet class so that it uses the additive distance measure.
  + Added a Facet::additiveDistance() method.
  + Added a Facet::euclideanDistance() method. (in case the user needs it)
  + Added a Facet::distance() method, which simply calls
    Facet::additiveDistance().
- Changed the Facet class so that it uses the additive error measure.
  + Added a Facet::dominatesAdditive() method, which uses the
    Facet::additiveDistance() method to check for eps-dominance.
  + Added a Facet::dominatesMultiplicative() method, which uses the
    Facet::multiplicativeDistance() method to check for eps-dominance.
  + Added a Facet::dominates() method, which simply calls
    Facet::dominatesAdditive().
- Changed BaseProblem::doChord() (only one line) so that it uses
  Facet::dominates() to check if a point is eps-dominated by the facet
  that generated it (instead of using Facet::ratioDistance()).
- Added the distance() and euclideanDistance() methods to the Hyperplane
  class.
  + Currently not using the class.
  + Not sure if I will delete the Hyperplane class or add it inside Facet.
- Removed the NotEnoughAnchorPointsException exception class.
  (wasn't using it anywhere)
- Moved all exception classes to the exception_classes namespace
  (sub-namespace of pareto_approximator).
- Moved utility.h's functions to the utility namespace (sub-namespace
  of pareto_approximator).
  + Didn't move the functions inside utility.cpp's anonymous namespace.
- Added the -W option to the examples. It's the same as
  the --without-exact-pareto-set option.
- Changed the EXTRACT_ANON_NSPACES option in Doxygen's configuration file
  (Doxyfile) to YES.
  + Doxygen will now extract members of anonymous namespaces and adds
    them to the documentation.
- Updated documentation.
- Updated tests.