Create metric contours dataset

Use `gdaldem contour` to create a 10m contour dataset, then export to GeoJSON seq, run through Tippecanoe with the same layer name as the USGS 40' contours (also make sure that the 50m lines have the right key so they can be made bold). 

Then it should be possible to switch effortlessly between an imperial and metric basemap, where the imperial basemap isn't just 40' converted into meters.