started documentation for PyViennaMesh
added requirement flag of parameters
added example
added index based interface for parameter query
added CMAKE_CXX_FLAGS to external projects
added parameter interface
Merge branch 'FlorianRudolf/fix-cmake' into next

Conflicts:
	CMakeLists.txt
	external/CMakeLists.txt
Merge branch 'FlorianRudolf/feature-parameter-interface-improvement' into next
Merge branch 'FlorianRudolf/feature-pyviennamesh' into next
removed examples
more documentation, fixes
added automatic documentation
Merge branch 'FlorianRudolf/feature-pyviennamesh' into next
Merge branch 'FlorianRudolf/feature-documentation' into next