CFL check

[deitrr]

Can we make THOR check the CFL condition, and yell at the user to decrease the time-step or tell the user when the code crashed because the time-step was too large?

[deitrr]

To elaborate: we will need to discuss with Joao how to get the CFL condition. At the moment, it seems to work to set the time-step < the radiative cooling time of the uppermost layer (I don't have the formula in front of me so I will add it later).