Optimized get_realizations a bit more

[micheles]

Here is the speedup for EUR (40%):

| calc_XXX, maxmem=201.8 GB  | time_sec | memory_mb | counts    |
|----------------------------+----------+-----------+-----------|
| building full_lt           | 2_031    | 84_778    | 1         |
| building full_lt           | 1_230    | 84_767    | 1         |