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.