Use a chrono::duration type as underlying time representation

[sansumbrella]

As demonstrated by the date library, std::chrono operations compile down to simple numeric operations without any extra type information. This means calculations with duration<double, seconds::period> are the same as calculations with double.

As a benefit, we would get explicit temporal information in our timestep.

  .rampTo(10.0f, Seconds(1.5))
  .rampTo(20.0f, Seconds(1.0) + minutes(2));

On a current project, I have a constexpr converting chrono types to double to pass through to Choreograph. Will be nice to skip that extra writing. Will, however, require explicitly defining the incoming time type.

[sansumbrella]

Alternatively, provide methods that perform the chrono conversion as needed (it might be annoying to always specify Seconds(1.5) rather than just pass in a number).

User-defined literals might alleviate, too, though their could be ambiguity with chrono literals. Consider sec and msec to disambiguate. Alternatively, defining the literal for double could cause our s suffix to take precedence.