Build a CollectionExtractor class to download all items in a (optionally filtered) collection

[nickrsan]

We have GroupedCollectionExtractor which iterates through a set of AOIs and downloads all the items in a collection for that AOI. It'd be nice to build out a simpler version that just extracts the collection, and takes care to avoid regridding when possible, by not clipping the bounds either to the bounding box or the strict boundaries of a geometry. If we just export the image as is (possibly extracting its footprint to use as our boundary, if necessary), then we should be able to align the grid closely to the source, or skip a regridding.

This should then run an EEDLImage extract for each item in the collection. I'd personally love the following features, but some of them might need to wait:

• Optionally, remerge the downloaded images from the collection after the fact - that is, run a second mosaicking operation on the final images. OpenET's imagery has multiple images for the same date that are spatially segmented, and others will too. We could use mosaic_by_date (which should remain an optional flag) to force Earth Engine to merge them, but that seems like it could introduce other problems, or at the very least, additional cloud compute. By default, it should dump each image as a separate item.
• Allow for specifying additional GDAL flags - this may be a broader feature. I'd like to process them into COGs at the same time as I mosaic them to skip a later step.
• Allow for similar sets of features to the GroupedCollectionExtractor in terms of running mosaicking and zonal stats - I anticipate that this may become a parent class of GroupedCollectionExtractor, but they might have different entry points. They'll share a few core pieces of code, as well as most of the attribute definitions. Some thought and care here will help.