Add eco endpoint for group filters

Currently this calculated the eco benefits for the entire urban
forest (since search hasn't been implemented yet).

The endpoint does the same extrapolation that OTMv1 does:

   calc_benefit = ...
   trees_with_benefit_data = ...
   avg_benefit = calc_benefit / trees_with_benefit_data
   extrapolated_benefit = avg_benefit *
                          (total_trees - trees_with_benefit_data)

   total = extrapolated_benefit + calc_benefit

The JSON result looks something like:

    {"benefits":
        {"energy": 256250.91907965578,
         "co2": 659479.0384184252,
         "airquality": 1596.7681393827293,
         "stormwater": 1685788.4340072556 },

     "basis":
        {"percent": 2.1934634788330774e-05,
         "n_total": 45590,
         "n_calc": 1}}

Where the variables in the basis section are:

    n_total: Total trees use to calculate the benefits
    n_calc:  Number of trees that had benefit data
    percent: Percentage of trees with benefit data