A step towards better Doxygen documentation for Simbody's vector and matrix classes.

- There is a new doxygen Module (like a "topic") on the vector/matrix stuff. The basic small and large matrix classes now belong to that module (using doxygen's `@ingroup` command).
- I extracted some text from OpenSim's SimTK matrix library topic and used it to seed the general discussion in the new module, but that needs to be reworked.
- I went through the Vector_ class and documented all the methods (many were undocumented). This needs to be done for all of these classes but you have to start somewhere!
- I added misc. documentation here and there.