DSE-Process: Add an ExplorationFeature service.

* The IExplorationFeatureService allows to add extensions to the DSE
  process that consist of sub-modules, e.g., Variables, Evaluations,
  or additional operations on existing encodings.
* While the IExplorationFeatureService is used to manage all existing
  extensions, the activated ones (configuration) is passed to the DSE
  by the exploration backend. Currently, the activated features are hard
  coded, but shall be moved to the UI.
* Consequently, the IExplorationModuleService has bee removed. Its
  functionality has been moved to the ExplorationFeature abstract class
  that are single features registered with the
  IExplorationFeatureService.
* The activated features now govern the installation of the sub-modules
  (Creators, Decoders), instead of the previous
  IExplorationModuleService.
* When setting up the Opt4J exploration, the activated decoder
  configuration is now checked for consistency to avoid circular
  dependencies and duplicate responsibilities of decoders to create a
  phenotype.
* An IExplorationFeature type is added to the MOEA ecore model to allow
  referencing exploration features from EMF classes.

Signed-off-by: Alexander Diewald <diewald@fortiss.org>