Implement Duration module to provide time conversion utility functions.

Is your feature request related to a problem? Please describe. Current time conversion using chrono is verbose. Also, time as a double isn't supported out of the box.

Describe the solution you'd like

• Define std::chrono::duration<double> as druid::core::seconds
• Implement functions to convert a std::chrono::stead_clock::duration:
  • druid::core::to_seconds() -> seconds
  • druid::core::to_milliseconds() -> std::chrono::milliseconds
  • druid::core::to_microseconds() -> std::chrono::microseconds
  • druid::core::to_nanoseconds() -> std::chrono::nanoseconds
  • Implement appropriate tests in Duration.test.cpp

Describe alternatives you've considered

I considered making this a class Duration with methods for the conversion but I think this a little nicer to maintain.

Additional context This is really needed for the seconds type we'll get from this. Most math / physics work off of fractional seconds.