Converted Object::print to be const along with a few other methods it invokes, in particular: updateXMLNode, AbstractProperty::writeToXMLParentElement
Fix signature of cpp print method in swig interface file to fix python wrapping test failure.
Propagate const correctness introduced in UpdateXMLNode to callers in Property and Object classes.
Update documentation to describe the use of _document and explain why const_cast. Also removed some commented out code block.
Fix typo and beef up comments per Sherm.
Merge pull request #191 from opensim-org/make_print_const

Make Object print method const