Add option to specify custom trained model for brain extraction

[sebastientourbier]

This new feature can be accomplished by:

• Addition of new arguments --custom_ckpt_brain_localisation and --custom_ckpt_brain_localisation in pymialsrtk/parser.py(where the parser of the BIDS App is defined) to specify custom paths (prefixes) to the files of trained networks. As we are using a container image, these files should be mounted and accessible inside it. I would suggest to use to code/ folder to put these files which is already mounted.

• Modification of docker/bidsapp/run.py and/or pymialsrtk/pipelines/anatomical/srr.py such that in_ckpt_loc for localization and in_ckpt_seg for segmentation are set to the custom paths (prefixes) specified by the new BIDS App arguments.

[hamzake]

This is incorporated in the two checkpoints to feed the node with (in_ckpt_loc for localization and in_ckpt_seg for segmentation)