Streamplot integration control

Two new options have been added to the streamplot function that give the user better control of the streamline integration. The first is called integration_max_step_scale and multiplies the default max step computed by the integrator. The second is called integration_max_error_scale and multiplies the default max error set by the integrator. Values for these parameters between zero and one reduce (tighten) the max step or error to improve streamline accuracy by performing more computation. Values greater than one increase (loosen) the max step or error to reduce computation time at the cost of lower streamline accuracy.

The integrator defaults are both hand-tuned values and may not be applicable to all cases, so this allows customizing the behavior to specific use cases. Modifying only integration_max_step_scale has proved effective, but it may be useful to control the error as well.