(to simplify; 2 flavors are enough)

- Documentation can now be built by command 'make doxyuser'
- configured doxy files now in build/doxygen (no longer under version
  control)
- html output now under html/build

This resolves issue http://apps.jcns.fz-juelich.de/redmine/issues/2385

- rename flag BORNAGAIN_RELEASE -> BORNAGAIN_CONFIGURE_DOXY
- write configured Doxyfiles to auto/Doxygen

issues are closed yet)

Doxygen: changed output dir to html, BORNAGAIN_PYTHON definition added, @brief propagated to class method list

Redmine: #1858
in Material class hierarchy