Filter based on value of state selector

[alaws-USGS]

• [x] Create a new branch.
• [x] Building on #474, use the value of the selected state to filter the following:
  • The state that is plotted
  • Only streamgages (#471) inside the states geometry should be displayed (check out geopandas clip
• [ ] Zoom the map to the extent of the filtered geometry.
• [ ] Add a button to clear the filtering and return the zoom to the contiguous US
• [ ] Push commits as needed and open a pull request when complete.