Make default get_dt_max error on invocation
The method get_dt_max inside BaseCase was added in a previous commit as part of the effort to include the 'default' timestep-checking code inside BaseCase. The intent is for user code to override this method with a problem-specific maximum timestep, but if the user code did not do this the previous implementation would return '0'. This would cause SPINS to error, as a timestep of <= 0 is obviously invalid. However, the error message would be unintuivie, referring to the <=0 timestep rather than the root cause of the user not implementing get_dt_max(). This change replaces the previous default method with one that immediately aborts, with an assertion failure if assert() is enabled at compile time.