download_feature() and Natural Earth (or equivalent)

[Huite]

Both the WFLOW and iMOD model generator need a land-sea definition to draw model boundaries. @visr and I download Natural Earth right now, but it would be very useful to get a clipped one from HydroEngine. I think this would require a get_feature function, cf. get_raster.

Additionally, for MODFLOW, basically everything is rasters. I rasterize the polygon above, then use it as a mask to generate input. It might be worthwhile to provide an additional function download_mask (or something) which clips the vector data, and rasterizes it on the fly to a given cellsize and crs.

Finally, there's the question of which feature data to use. Natural Earth is public domain, so it shouldn't be an issue; Martijn suggested a dissolved USDOS/LSIB/2013, which is already available on EarthEngine. ee.FeatureCollection('USDOS/LSIB/2013')