Parallel data loading

[atrabattoni]

When loading from one disk, making parallel data request usually does not improve I/O speed. But in some parallel hardware context or if the data need to be inflated (e.g. because ZFP compression was used) parallel loading of data chunks can improve I/O significantly (i.e. 8x speedup has been observed).

Two Xdas mechanisms could be improved:

• The __array__ method of the VirtualArray class could have a parallel optional argument that could also be configured by xdas.config.set("parallel_read", 8).
• The DataArrayLoader class could load several chunks in a parallel fashion also with a parallel optional argument (right now it only loads chunks one by one in a async way).

It would be nice to do some benchmarks to se what is the best, probably implementing the two is the best.