Adds new EventEffect - moves the target agents into specified state

[curds01]

This builds on top of #110. This needs rebasing after that PR merges.

The new EventEffect comes with an example and a dedicated README.md discussing the example.

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This change is 

[MengeCrowdSim]

Minor nits

Reviewed 13 of 13 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @curds01)

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src/Menge/MengeCore/Agents/Events/change_state_effect.h, line 22 at r1 (raw file):

/**
  @file   change_state_effect.h
  @brief  The definitin of event effect that moves the target agents to a new state.

BTW typo: definition

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src/Menge/MengeCore/Agents/Events/change_state_effect.h, line 83 at r1 (raw file):

protected:
  /** @brief  Implements EventEffectFactory::setFromXML().  */
  bool setFromXML(EventEffect * effect, TiXmlElement * node, 

nit: new style Class*

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src/Menge/MengeCore/Agents/Events/change_state_effect.h, line 89 at r1 (raw file):

  EventEffect * instance() const override { return new ChangeStateEffect(); }

  /** @brief	The identifier for the "state" string attribute.  */

nit: tabs

[MengeCrowdSim]

Reviewed 1 of 1 files at r2.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @curds01)

[MengeCrowdSim]

Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @curds01)

[MengeCrowdSim]

Reviewable status: complete! all files reviewed, all discussions resolved