Typos and Noop Default in Callback

[SSoelvsten]

• Fixes spelling error of unknown (the second n is missing); this is a breaking change.
• Fixes some spelling errors in comments.
• Makes the default implementation of the Callback class lots of noops. This way, one only has to overload/implement the few relevant functions.

[vaughnbetz]

Thanks, and sorry I didn't notice this until now. When you say this is a breaking change, does anything in VTR (users of the parser) break?

[vaughnbetz]

@soheilshahrouz : while you're checking out the make format on libblifparse, could you also test this change works fine with VTR? If so I'll merge it.

[SSoelvsten]

No worries about the delay; thanks for this parser - it has been great help for my research. 🙂 If a user of the parser makes use of the LogicValue enum (which I'd imagine they would), then compilation breaks. If bumping the major version number is not desirable, then one can exclusively merge 9771095 in instead.