Add code for SSPMS43 and update/correct computations for SSP LMM.

In this PR:

1. Correct solution update for SSPMS32.
2. Add code for SSPMS43.
3. Update/clean code for setting ```cfl_max``` and ```dt_new``` for SSPMS**. Now registers for previous steps only need to store previous solution and time-steps. ```cfl_max``` and ```dt_new``` are functions of the SSP coefficient which is being calculated each step based on previous time-steps.
4. ```nregisters``` now are exactly the number of registers required by each method.
5. Fix bug for LMM with PETSc. (correctly creating stage objects used for stage evaluations for the starting RK method (SSP104)).
6. Update nosetests for SSPMS32.