Support polygon input

[ejm714]

Goal: support users who want estimates across an entire body of water (e.g. heatmap-like output)

This would entail a preprocessing step where we:

• accept a polygon as input
• create a grid at Sentinel-2 resolution
• create an input csv where the lat/lons correspond to the centroid of each grid cell

Likely implementation

• we allow a csv where the rows are date and polygon to support estimates across multiple water bodies at once
• TBD what the output is (e.g. a raster vs. csv)