Parallelize classical PSHA calculator for logic tree samples

[Damiano Monelli]

Currently the classical calculator is parallelized in terms of sites only, that is given a logic tree sample, the set of sites for which hazard curves must be calculated are distributed over multiple task.

However, in case of multiple logic tree samples and a single site, calculations are done sequentially, because for each tree sample only one task is executed.

The classical calculator should be parallelized also in terms of tree samples so that multiple tasks for different samples can be executed in a parallel way.