split the dem-fusion code snippets into separate script files

minor cleanup of code comments

updated boundary assessment code to use multires spline results

fixed critical typo in post-processing enblended layer

added R code to create various boundary assessment figures

added R code to 'enblend' DEMs (multires splines, Burt & Adelson, 1983)

superficial tweaks to DEM fusion R code

separated out code to profile slope near 60N boundary

added slope boundary descrepancy plotting code

added GDAL/R code for fusing and correcting DEMs near Canadian border