Add a default bounding box for usgs-elevation

[kkothari2001]

#1111

Sample runs for bounding box:
retriever install postgres usgs-elevation -b -94.98704597353938 39.027001800158615 -94.3599408119917 40.69577051867074
retriever install postgres usgs-elevation -b -94.9 39.02 -94.4 40.6

We should be able to run retriever without the bounding box arguments.