Skipping useless calculations

[micheles]

Restored a feature that was lost in #9103. The improvement on the Chile calculation is minor in general but significant in "computing gmfs":

# now
| calc_54950, maxmem=0.5 GB | time_sec | memory_mb | counts    |
|---------------------------+----------+-----------+-----------|
| total gen_event_based     | 159_183  | 29.7      | 10_014    |
| computing mean_stds       | 66_799   | 0.0       | 1_224_412 |
| instantiating GmfComputer | 51_591   | 0.0       | 1_228_686 |
| computing gmfs            | 12_768   | 0.0       | 1_477_313 |
| updating gmfs             | 9_868    | 0.0       | 2_701_725 |
| EventBasedCalculator.run  | 1_139    | 143.7     | 1         |

# before
| calc_54946, maxmem=0.5 GB | time_sec | memory_mb | counts    |
|---------------------------+----------+-----------+-----------|
| total gen_event_based     | 165_493  | 29.2      | 10_014    |
| computing mean_stds       | 67_199   | 0.0       | 1_224_412 |
| instantiating GmfComputer | 51_612   | 0.0       | 1_228_686 |
| computing gmfs            | 19_084   | 0.0       | 3_573_951 |
| updating gmfs             | 9_921    | 0.0       | 4_798_363 |
| EventBasedCalculator.run  | 1_195    | 144.6     | 1         |