Add timeslider append mode

[spencerwahl]

Related Item(s)

https://github.com/ramp4-pcar4/tcei-tmx-cwa-storylines/issues/126

Changes

• [FEATURE] Adds 'append' mode to timeslider and ability to configure the play mode
• [FEATURE] Interval between timeslider animation steps is configurable

Notes

New animation config snippet example:

"animation": {
    "playMode": "append",
    "interval": 500
}

Testing

Steps:

1. Scroll down to the map with a timeslider
2. Press play and see only the right handle move (and faster than normal!)
3. See data show up on map but not disappear (until it loops)

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This change is 

[github-actions]

Your demo site is ready! 🚀 Visit it here: https://ramp4-pcar4.github.io/story-ramp/timeslider-enhance/#/en/00000000-0000-0000-0000-000000000000

[yileifeng]

Reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @spencerwahl)

[RyanCoulsonCA]

Reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @spencerwahl)