Remove integration time as a termination condition in vector field planner

[mkoval]

From @mkoval in #268:

Integration Time. The time parameter used by the integrator is not meaningful because the planner does not consider dynamics. Therefore, integration time should be treated as an implementation detail of the integrator, i.e. not used as a termination condition or argument to the vector field. This may require calling the Stepper directly, rather than using the integrate_adaptive() helper method. Thankfully, that method does not actually do very much.