when state_type == time_type (e.g. 1d odes with state_type = double), some
do_step overloads are disabled due to ambiguities of parameter structure.
However, the initialization of the Adams-Bashforth needs some of those
disabled functions in its initialization. As a fix, I added do_step_dxdt to
the stepper base classes to provide direct access to the required functions
that will not be disabled in the case of state_type == time_type.
