Additional compositing functions - mediod, best available pixel, others?

[mikemahoney218]

Right now, the compositing methods in get_stac_data() are pretty simple -- defensible! But simple. It would be nice for additional compositing methods to be available.

Right now, I'm aware of two general approaches that would be nice:

• Using mediod pixels,
• Using the "best available pixel" (BAP) approach

I'm not sure if there are good R implementations for calculating either, or if this is going to require writing implementations in rsi. If the latter, those implementations should be user-facing.

I'm opening this issue as a signpost for a new feature I'd like to add for the second CRAN release of the package (I have no intention of including it in 0.1.0). If anyone reading this has other compositing methods they'd like to see added (or knows of efficient implementations I can borrow!) please leave a comment!