fix: generic dem tile names for SW hemisphere

[edparris]

This change fixes an error in how the DEM file names are calculated by the GenericDEMTileSet. The original code did not handle coordinates in the South or West hemispheres correctly (negative decimal degrees values). The DEM tile 044W contains elevation values for coordinates [-44.0, -43.0) so a coordinate value of -43.5 needs to be floored to -44. That is the natural behavior of the python floor() operator but the original implementation was relying on the degrees, minutes, seconds calculation in the GeodeticWorldCoordinate format operator which converts the decimal value to a positive number before truncating the degrees. This change floors the number before passing it into the format string ensuring the correct decimal value is chosen for a DEM tile. Unit tests have been updated to include the failing example that brought this issue to our attention.

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