Put stronger static asserts in path element types.
Put stronger static asserts in path element types.

The key was putting the inline implementation of the (templated) base's
constructor after all the derived declarations.

leemichaelRazer/OSVR-Core,feilen/OSVR-Core,OSVR/OSVR-Core,godbyk/OSVR-Core,leemichaelRazer/OSVR-Core,feilen/OSVR-Core,leemichaelRazer/OSVR-Core,godbyk/OSVR-Core,OSVR/OSVR-Core,godbyk/OSVR-Core,d235j/OSVR-Core,godbyk/OSVR-Core,Armada651/OSVR-Core,godbyk/OSVR-Core,OSVR/OSVR-Core,godbyk/OSVR-Core,d235j/OSVR-Core,feilen/OSVR-Core,d235j/OSVR-Core,d235j/OSVR-Core,feilen/OSVR-Core,d235j/OSVR-Core,OSVR/OSVR-Core,OSVR/OSVR-Core,leemichaelRazer/OSVR-Core,OSVR/OSVR-Core,feilen/OSVR-Core,Armada651/OSVR-Core,leemichaelRazer/OSVR-Core,Armada651/OSVR-Core,Armada651/OSVR-Core,Armada651/OSVR-Core