openFile of CST_MicrowaveStudio class should be a hidden method

Calling openFile does not update the COM objects of all the sub-classes composing CST_MicrowaveStudio. Therefore, if an user tries to open an existing project using directly this method after having closed a former project (opened by the init method of the class), any further manipulation of the project will fail since it will try to make use of the former COM objects.

❗ Please note that after making this change it will be necessary to update also any other file (notably tests) making use of this method.