Removed "Dictionary<string, object> data" in ModelEventArgs

#### Background
This is mainly to resolve this task of mine (which is part of a bigger command framework task). For more technical details please refer to the following task:

http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-198

#### Unit tests
No new code has been introduced, and test cases that are affected are still passing: DynamoCoreUITests, DynamoCoreTests, DynamoPythonTests and DSCoreNodesTests.