Related to tickets: #2784 #2786 #2857 #2858 #2859
that are subtickets of #2677

Changes: (ticket number)
 - All docstrings have been completed in a standardized format. (2784)

 - Method load_fmu2 renamed to load_fmu. (2786)

 - Method reset_slave in FMUModelCS2 also resets time and clears the log. (2857)

 - Both FMUModelME2 and FMUModelsCS2 constructor methods checks if the model
   needs an execution tool and if so, raises FMUException. (2858)

 - Method instantiate_slave changed FMIL-function argument from empty     string to NULL. (2859)

Also the following changes are made:

 - Get/set and (de)serialize FMU-State-methods raises notImplementedError.

 - Method debug_logging logs all categories by default. Raises Exception if
   logcategories are defined by the user.

 - Method get_directional_derivative is moved to class FMUModelBase2.

 - Unnecessary comments have been removed.