Optimized even more the calculation of the GMFs

[micheles]

By using numba and by vectorizing. Here are the results for Albania on oq1, before/after (1.2x):

| calc_54932, maxmem=0.6 GB | time_sec | memory_mb | counts  |
|---------------------------+----------+-----------+---------|
| total gen_event_based     | 27_004   | 133.8     | 645     |
| total gen_event_based     | 22_969   | 146.0     | 645     |

In most cases the improvement is even better. For instance in Canada with ebrisk is an 1.6x:

| calc_54937, maxmem=3.6 GB    | time_sec | memory_mb | counts  |
|------------------------------+----------+-----------+---------|
| total ebrisk                 | 12_307   | 131.2     | 646     |
| total ebrisk                 |  7_613   | 136.8     | 646     |