docu: doxygen-related fixes

explicitly use Doxygen commands
docu: docstrings for Ecapsulation
docu: docstrings for VectorEcapsulation
docu: enable JAVADOC_AUTO_BRIEF

and got rid of `@brief` and `@details` sections
docu: using MathJAX markers directly (not Doxygen's)
Merge pull request #27 from torbjoernk/feature/further-docu

some docu fixes and additions