Improved build_global_exposure/4

[micheles]

Part of #9227. By using pandas directly on small files I get much faster:

Received 16291 * 472.46 KB {'tot': '7.34 GB'} in 209 seconds
<Monitor [michele], duration=294.58053159713745s, memory=13.18 GB>

| calc_56312, maxmem=28.8 GB | time_sec | memory_mb | counts |
|----------------------------+----------+-----------+--------|
| total exposure_by_geohash  | 1_667    | 26.5      | 3_365  |
| total gen_tasks            | 444.2    | 6.26842   | 13_001 |

| operation-duration  | counts | mean | stddev | min     | max   | slowfac |
|---------------------+--------+------+--------+---------+-------+---------|
| exposure_by_geohash | 75     | 22.2 | 23%    | 0.25935 | 31.2  | 1.40277 |
| gen_tasks           | 19     | 23.4 | 180%   | 0.33477 | 152.3 | 6.51578 |