Fix increment of _occuredElementList in XmlSequenceParser.cpp

[rgourova]

Reference to a related issue in the repository

Related to issue #209

Add a description

The proposed changes fix a bug related to _occuredElementList in XmlSequenceParser::XmlSequenceParser. The change introduces a check if _occuredElementList contains a parser, and, if it does, the number of times the parser has been encountered is incremented, otherwise a new key is created in _occuredElementList for the parser.

Added a new scenario (cpp/applications/openScenarioTester/res/exampleFilesVersion1_2/FollowTrajectory_Nurbs.xosc) to test the changes.

Check the checklist

• [X] My code follows the contributors guidelines of this project.
• [X] I have performed a self-review of my own code.
• [ ] I have made corresponding changes to the documentation.
• [X] My changes generate no new warnings.
• [X] I have added tests that prove my fix is effective or that my feature works.
• [X] New and existing unit tests / travis ci pass locally with my changes.