In "run_vadere_console_with_all_scenario_files.py", use "os.path.join()" instead of hard-coded platform-specific path separator

In ".gitlab-ci.yml", deleted outdated comments and made stages "unit_test" and "integration_test" more robust

VadereGui/src/org/vadere/gui/onlinevisualization/view/OnlineVisualisationWindow.java:
- Fixed bug: The scroll pane was not added to the underlying model.
  Therefore, it was not possible to capture mouse wheel events.

add tests to solve the eikonal equation on the unstructured mesh for 2 scenarios (bridge and filled chicken).

Added "AbsorbingArea" and "TargetChanger" to "TopographyIterator" to make them selectable in PostVis

In "TargetChangerController", added optimization to avoid unnecessary calculations of dynamic floor fields

In top-level "pom.xml", use new versions for "commons-configuration2" and "commons-beanutils" to avoid logging a stack trace

In "ProjectView", avoid exception by testing if value for config key ""History.lastUsedProject" is the empty string

"TargetPedestrian.getId()" is now used by "TargetChangerController" to implement follower behavior.

Furthermore, deleted unnecessary code in following files:
- VadereSimulator/src/org/vadere/simulator/models/osm/OptimalStepsModel.java
- VadereSimulator/src/org/vadere/simulator/models/osm/PedestrianOSM.java

Fixes issue #267.

VadereSimulator/src/org/vadere/simulator/models/osm/OptimalStepsModel.java:
- Added TODO to use new list "Agent.getFollowers()".