convert to handle class

[erik-roberts]

this enables mdd objects to be passed by reference instead of passed by value, thus saving a lot of memory for large objects. this also allows objects to be modified without having to return the object. finally, it allows listeners/events/callbacks in subclasses, which would be useful for gimbl-vis. if it’s not a handle class, no subclasses can be handle classes.

TODO:

• look for places in need of copy method usage to make separate object.
• fix methods with own file

[erik-roberts]

started in branch iss67_handle_class

[erik-roberts]

an alternative that i just thought of, that I'm trying to implement in GV, is to use a wrapper handle class with dependent properties that access mdd. we could make this part of the MDD library, so that people have the object of a normal or handle object, for pass by reference vs. pass by value.

finished this idea in #69
you can check it out by using MDDRef class