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.