Timechart refactoring

[51-code]

Fixes #473 .

Refactored TimechartStep:

• Made it immutable
• Removed the AbstractTimechartStep class which basically just contained getters and setters and the class variables

Reworked TimechartTest:

• The tests broke because they checked the correct translation of the parse tree with the getters from TimechartStep. Reworked the tests to assert equality with an expected TimechartStep object instead.

Refactored TimechartTransformation:

• Removed unused parameters, unused code, commented out code and so on.
• Refactored the visiting logic. Old version used a for loop to go over the child nodes and collect values for the TimechartStep. The new version calls visitChildren() and collects the values for TimechartStep to class variables instead.

[eemhu]

Check the possibility of using ContextValue to avoid object mutability.