Change _rk_stages to _registers
Add SSPMS(3,2) and general linear multistep time-discretization.
Edit linear multistep methods.
Adding variable time-step functionality to SSPMS32, editing LMM, adding a dictionary for ssp coefficients in sharpclaw class.
Corrections so that classic solver runs.
Change _rk_stages to _registers
Add SSPMS(3,2) and general linear multistep time-discretization.
Edit linear multistep methods.
Adding variable time-step functionality to SSPMS32, editing LMM, adding a dictionary for ssp coefficients in sharpclaw class.
Corrections so that classic solver runs.
Introduce generic functions for cfl_max and dt_new.
Merging from ketch/linear_multistep_methods.
Adding sspcoeff = None for 'RK' and 'LMM' so that nosetests run.
Set default cfl_desired and cfl_max to 'None'. Values are assigned in setup according to time-discretization.
Add tests for SSPMS32 method (for advection 1D, acoustics 1D and 2D).
Set default time_integrator to 'SSP104'.
Remove return in _allocate_registers.
Remove unnecessary line.
Don't fail if the Riemann solver is not in static.py

Also, provide better error messages when a solver is not valid.
Remove duplicate code for SSP104 computation.