Tree model.
Added basic function handling.
Fixed function call in expression evaluation.
Merge branch 'feature/model-obj' into develop
Added test for function store.
Renamed test file.
Added utility methods for store. Added function name in the function interface.
Merge branch 'feature/test-function' into develop
Test for expression trees
Throw exception if function don't exists.
Exceptions that can be thrown.
Test exceptions.
Merge branch 'feature/function-exception' into develop
Documented the exception classes.
Added package info.
Merge branch 'feature/javadoc' into develop
Made expression tree interface an abstract class.
Added toString implementation.
Added exception to be thrown when parsing fails.
Tree parse tests.