Add riemann-invariant, characteristic bcs (Ref. A. Jameson & T. J.  Baker, Paper 83-1929, AIAA)

	modified:   pyfr/solvers/euler/inters.py
	new file:   pyfr/solvers/euler/kernels/bcs/char-riem-inv.mako
	new file:   pyfr/solvers/euler/kernels/bcs/slp-adia-wall.mako
	modified:   pyfr/solvers/navstokes/inters.py
	new file:   pyfr/solvers/navstokes/kernels/bcs/char-riem-inv.mako