PostgreSQL function to return parks ordered by distance and within radius

[nealf]

@jhawley We'll implement this and it should be available through the PostgREST api when it is done.

[nealf]

This function should be available at the endpoint https://parks.api.codefornrv.org/rpc/get_radius

Function calls in PostgREST require a POST with named variables, which in this case are:

• longitude
• latitude
• radius (in miles)

[nealf]

Duplicate of #13