Add command line option for deriv_root

[vferat]

As I develop a Docker App using mne-bids-pipeline, I want a straightforward way to overwrite the bids_root and deriv_root values defined in the configuration file from the command line. This enhancement aims to simplify Docker volume mapping.

Currently, the option to override bids_root is already available (source), but there is no similar option for deriv_root.

I suggest adding a command-line option to the mne-bids-pipeline tool to allow users to specify the deriv_root directory. This behavior is similar to other BidsApps such as FMRIprep and MRIQC, where command line arguments overwrite configuration files.

If you are interested, I would be happy to contribute by taking care of the pull request.

[welcome]

Hello! 👋 Thanks for opening your first issue here! ❤️ We will try to get back to you soon. 🚴🏽‍♂️

[larsoner]

Makes sense to me, PR welcome!