use more than one CPU at data preparation

[NJSturm]

Motivation : predictions of large batches of structures would benefit from having the possibility to use more than one CPU at data preparation mainly to gain speed. Melloddy-Tuner prepare to predict functionality has multi-process enabled.

However, the number of CPUs to use for the data preparation seems to be hard coded to 1 here : L67.
Would it be possible to have an PreparedData object attribute allowing to control the number of CPU used for data preparation ?

[Fabien-GELUS]

Hello, sure, you found the right place in the code, you can simply put the num_cpu argument in the init. Feel free to open a PR, I will review it :)

[wheyndri]

btw, this is only working with num_cpu=1 as it stands due the following multiprocessing error:

RuntimeError:
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

[adarbier]

Hello,it seems that having num_cpu>1 requires some modification in Melloddy-Tuner