Examples: Add NASA GIBS data example

[gkjohnson]

The NASA GIBS weather urls are formatted like so in WMTS format:

// time = 2015-08-02T
// matrix set = 250m
// levels = 9
`https://gibs.earthdata.nasa.gov/wmts/epsg4326/best/MODIS_Terra_CorrectedReflectance_TrueColor/default/{time}/{matrix set}/{z}/{y}/{x}.jpg`

Service supports daily data sets at 512 x 512 pixels per tile.

Demo

• Animate date with fade interpolation between two target data sets. Unload the current and preload the next dataset and fade to it (see avoiding flickering in ImageOverlayPlugin: Improvements #1206)
• Use a simple globe underneath (possibly add "only geometry" option to load the tile structure without image overlays)
• Improve the tiled image level-mapping logic (ability to specify the target meters-per-pixel value or auto infer?)
• Black under-texture