Added Dynamic step size feature

[teubert]

Allows user to specify a function f(t, x) -> dt that specifies the next step.

This enables use case where step sizes are dynamic (i.e., a function of time or state). For example: step size my decrease as you approach an event state

[kjjarvis]

This is a great new addition. The implementation in prognostics_models is straightforward and clear, and dynamic_step_size provides a detailed and easy-to-follow example.

[GGorospe]

New example with dynamic step size looks great!