Fix OpenSim namespace doxygen.

This restores the documentation for many classes. This addresses #144.

I also changed the header guards in most of these files.

In general, I did *not* edit the documentation itself. It is possible that I am
now exposing *incorrect* documentation. I fixed at least one documentation
mistake, but did not look for errors.
Fixed typo in comment.
Fix header guards to be underscore-separated.
Merge branch 'namespace-docs' of github.com:opensim-org/opensim-core into namespace-docs
Fix typo (uncouupled).
Merge pull request #167 from opensim-org/namespace-docs

Fix OpenSim namespace for doxygen. Addresses issue #144 and updates header guards which were non-standard.
Create a CMake Config.cmake, etc.
Fix bugs with cmake config files.
Fix Config file lepton include path.
[cmake] Need newer CMake for ConfigVersion macro.
Fix lib path for windows.
[cmake] Use different macro for ConfigVersion.
[cmake] Remove accidental inclusion of ApiDoxygen.
[cmake] Update sample CMakeLists.txt.
Plugins now use the new OpenSimConfig.cmake.

Also the plugins weren't compiling. Fixed the errors and loosely checked
that the results were close to the expected results.
Merge pull request #187 from chrisdembia/cmake-config-files-2

Provide OpenSimConfig.cmake file
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.