[API] Use ParserUBB in ExtendedBoundaryHandlingFactory

This is a follow-up to !93 (merged). If somebody uses the ExtendedBoundaryHandlingFactory to read boundaries from a config file, they will now automatically use ParserUBB instead of UBB. This makes the new functionality more accessible.

For feature parity with lbm::UBB, I implemented the buffer packing/unpacking methods. I also fixed an incorrect signature of my constructor (ParserUBB::ParserUBB::ParserUBB instead of the correct ParserUBB::ParserUBB), which no compiler bothered to complain about. Also, I implemented the unregisterCell method so we don't unnecessarily keep around shared_ptr<Parser>s.