Allow configuration of duration multiplier for flyTo

[TheNumenorean]

Checklist

• I've searched through the plugins to make sure this feature isn't already available, or think it shouldn't require a plugin.
• I've searched through the current issues to make sure this feature hasn't been requested already.
• I agree to follow the Code of Conduct that this project adheres to.

Motivation

I really like the flyTo animation, but it can be too slow (>8 seconds). It is configurable with duration, but that doesnt scale to the distance, so it can be too fast or too slow.

Suggested solution

Add a "durationMultiplier" option to PanOptions, which when present replaces the .8 here..
And/or add a "maxDuration" option to PanOptions which when present is used as an upper-bound on the number generated by the above linked line.

Alternatives considered

I could calculate the duration myself based on distance, but I would essentially have to duplicate the calculations the function is doing.