Metadata and DEM fixes

- Fixed error with DEM not hiding and not fitting into the legend.
- Restructured metadata error handling to be more responsive to different levels of XML documentation/completeness
- Metadata now retrieved from http://carto.gis.gatech.edu/GCAMP/metadata/XXXXX.xml (this is where the main github repository is synced. So any changes/additions to metadata via github will be reflected there)