Update water/shoreline masking and integrate in dem_mask

[dshean]

I started this process with the HYDROBasins products for lakes/rivers (https://github.com/dshean/demcoreg/blob/master/demcoreg/get_hydro.sh) but never actually added support in the dem_mask functionality. Should be pretty straightforward - can use the RGI polygon handling as a template, or better yet, update with latest GDAL/rasterio rasterization approaches.

Should also add global shoreline vector data handling: https://shoreline.noaa.gov/

Upon quick inspection, the global NGA product might be good (https://dnc.nga.mil/PrototypeGSD.php). Though if we can find a shoreline dataset that can be pulled in on the fly, that would be better than another set of shp.zip that we download and combine...

Ultimately, we would like to have better support for classification of corresponding orthoimages (if available for the DEM, i.e. from stereo satellite imagery), mapping actual distribution of water, veg, snow/ice, but hard to have a general solution for this when supporting DEMs from many sources.

[dshean]

250 m static water mask from MODIS... https://lpdaac.usgs.gov/products/mod44wv006/

Maybe best to start with something like this, and focus on masking larger water bodies, ignoring the smaller, likely more ephemeral features for now.