Enhance the BIL to Parquet elevation utility to accept and stitch together multiple files

[thurber]

Since the HYDROSHEDS North American file does not quite cover the entire CONUS domain, we need to update the BIL to Parquet utility to enable it to process and combine multiple BIL files into a single reprojected and resampled output, such that it can combine the HYDROSHEDS NA and CA files.

Additionally, a unit test should be added.