Fixed XY-ising-model.py doc typo.
Tweaked a test to use BaseChain::setOptimizerSiteFailureToThrow()
Added a utility method to BaseChain to peak at the state site data.
Added Heisenberg model test.
Fixed fold marker and vestigial whitespace in boundaries.hpp.
Now there is a catch-all clause (std::exception) in nutcracker.
Added Doxyfile.in to .gitignore.