Base classes need virtual destructors

[ethanmusser]

Discussions in #39 pointed out that some of our inherited classes may not have a virtual destructor. As discussed here (in the Virtual Destructors section), virtual destructors are necessary in base classes to permit appropriate destruction of derived classes through pointers to their base.

TODO:

• [x] Add virtual destructors to all inherited classes which do not have them.

[ShaneRozenLevy]

I think the biggest improvement here is that it will probably stop the code from seg faulting on exit.

[ethanmusser]

Actually I think we unintentionally fixed that at some point along the way, the code doesn't seg fault on exit anymore. But a good preventative measure nonetheless.

[jdcaporale]

89 Did this.