Add support for variable time deltas

[mtilda]

Changes

• We add a new method Pid::next_control_output_with_dt() which handles a variable time delta (dt).

• The new method is called internally by Pid::next_control_output().

To do

• Update documentation:

  pid-rs/README.md

  Line 73 in 55f0ee2

  * Measurements occur at equal spacing. (`t(i) = t(i-1) + C`)

• Add test cases that use non-one values of dt.

[mtilda]

Checkout mtilda#2 to see how we might add a feature flag.

[braincore]

I agree with not breaking the existing signature for next_control_output. However, mtilda#2 seems overkill. Having a fn signature change as a result of a crate feature should be reserved for extraordinary cases.

How about you rename your new function to:

next_control_output_with_dt(&mut self, measurement: T, dt: T)

And it can simply coexist?

[michael-p]

This would be a super-nice addition to the crate! Hoping this can be merged soon, thanks for your work!