fix: make dt optional

[fibonacci61]

This PR updates the control loop traits to accept dt optionally, so that None can be passed in its place upon the loop's first iteration where there is no delta time. In addition, the motion algorithms have been updated to pass None.

[tropicaaal]

I don't think this is necessary? Every algorithm in evian::motion will start with a delay, meaning dt should never end up being zero or near-zero, and always reflect the cycle time of the loop. For the same reason, we pre-compute initial values of any delta so that prev_val in a motion algorithm always has some meaning (which prevents division-by-zero and other nasty things on the first loop iteration). I think that encouraging motions to be written this way versus just passing None has other advantages too.