Added RK3 SSP method

Solver:type rk3ssp

3rd order explicit RK scheme, Strong Stability Preserving

www.cscamm.umd.edu/tadmor/pub/linear-stability/Gottlieb-Shu-Tadmor.SIREV-01.pdf

Spatial TVD schemes can become oscillatory if discretised
in time with a non-SSP scheme. These schemes aim to retain
the stability properties of the Euler method, but with higher
accuracy.
Adding RK3-SSP solver