More improvements to Matrix/Vector documentation

- updated Doxyfiles to doxygen 1.8.5
- added currently-unused images directory to SimTKcommon/doc to avoid doxygen warning
- removed never-implemented "DeadMatrix" classes since they made the documentation confusing
- removed the MatrixCharacteristics module since it is only of internal interest and is confusing
- defined __cplusplus in Doxyfile; some declarations were being missed (including SimTK::Real)
- changed formal element type template parameters from "T" to "ELT" for consistency