Nektar++Config cleanup. This depreceates NektarCommon for external projects.

Instead, the cmake default way of handling external dependencies is now
recommended:

  FIND_PACKAGE(Nektar++ REQUIRED)
  ADD_DEFINITIONS(${NEKTAR++_DEFINITIONS})
  INCLUDE_DIRECTORIES(${NEKTAR++_INCLUDE_DIRS} ${NEKTAR++_TP_INCLUDE_DIRS})
  LINK_DIRECTORIES(${NEKTAR++_LIBRARY_DIRS} ${NEKTAR++_TP_LIBRARY_DIRS})
  TARGET_LINK_LIBRARIES(${ProjectName} ${NEKTAR++_LIBRARIES} ${NEKTAR++_TP_LIBRARIES})

see templates/executable for an example. The NEKTAR++_TP_XXX variables can be omitted if
the external CMakeLists.txt takes care of findig Nektar++ dependencies itself.