Add clip information to metadata while preprocessing.

[ethanplunkett]

Flag

• metadata$clip$clipped logical flag to indicate that it is clipped.
• is_clipped() function to return flag value if it exists and NA if it doesn't exist.

Clipped area

Possibly save data on clipping polygon. Considerations:

• Do not use terra's vector format as it contains pointers/environments that will break things.
• sf objects might be OK but I'd like to avoid any additional internal classes.
• Could save the clip in raster (matrix) format with cells that align with the output projection. I liked this until I realized that we'd only retain the part of the clip that overlaps the specie's distribution.
• Possibly save just the polygon's point coordinates in the final CRS - could get ugly with multi-part + holes.
• See if sf objects can be converted to standard data frame or matrix in a way that can easily be reversed. I think this is the best option

I think using a simplified representation of the sf polygon geom that can be easily converted back into an sf object makes the most sense:

• metadata$clip Vector representation of clipping polygon - ideally without using any non-standard R classes by converting the sf geometry into a three column data frame with columns: id, x, and y.
• metadata$clip$percent_lost Record the percent of each week's distribution that was lost during clipping (52 week vector).
• get_clip() new function retrieves the clip and converts it into a proper sf object with CRS assigned from geom$crs