Merge tag 'v0.2.4' into develop

Add Linear SSP Runge-Kutta class of schemes

Add Linear SSP Runge-Kutta class of schemes: implemented both
s-stage/s-1 order and s-stage/s order methods.

The class provides schemes for any formal order: the coefficients are
computed by means of recursive algorithm (with an original implementation)
that does not allocate unnecessary arrays...

Stable release, fully backward compatible.

Author: szaghi