[ENH] Expose n_procs option to Adni2BIDS Oasis2BIDS and Aibl2BIDS

[NicolasGensollen]

Closes #1007

[ghisvail]

This approach looks good and should work.

Let me propose a different alternative, which might reduce the amount of code duplication:

• Use Clinica's n_procs Click option wherever needed.
• Put the logic for computing the default value for n_procs in the default parameter for the option.
• Expose parameterization with n_procs to converters with a safe default value of 1, instead of making it optional and duplicating calls to get_n_procs to each pool.
• Plugging the n_procs option to the parameter should provide the benefits of safe defaults inside Clinica's code, whilst keeping backward compatibility at the CLI layer.

[NicolasGensollen]

Thanks for the review @ghisvail !
I implemented something that should go in the direction you detailed.
Let me know if I misunderstood or if you see some points to improve further.

[ghisvail]

Exactly what I had in mind 👌

I expected you would reuse the existing n_procs in cli_param to minimize the diff, but why not. I added a comment regarding the optional vs keyword param with default value.

[NicolasGensollen]

Thanks for the review ! 👍
The failures are unrelated, so I'll merge this one.