Min max playback rate

[mattjuggins]

See #3990

• Separate playbackRate into a min and max value. This means the application of rates can be the same between ServiceDescription and dash.js settings.
• Applies playback rates of value 1.0 if explicitly set.
• Modify catch-up algorithms to take into account these separate playback rate values.
• Update sample to take into account min and max playback rate values.

Some additional thought may be required on default playback rate fallback values, and whether to limit the range of possible values (again, see #3990).