[ENH] `singularity run`: handle FreeSurfer license and Templateflow home

[zhao-cy]

In this PR, I've made changes for how FreeSurfer license gets passed into container (breaking change! @djarecka ) and how Templateflow home path gets passed into container when container running. In details:

• FreeSurfer:
  • Now, users need to provide the exact path of FreeSurfer license when --fs-license-file in babs_singularity_run section in the container's configuration YAML file. The placeholder $FREESURFER_LICENSE is no longer recognized by BABS.
  • For singularity run that babs-init generates, this license path will be bound. This file won't be copied to BABS project folder anymore.
• Templateflow:
  • There was actually no bug, but I made enhancement of how the path in TEMPLATEFLOW_HOME env variable is passed into the container. Now, --env is used.

Currently, the generated singularity run looks like below (if --fs-license-file is one of the arguments, and env variable TEMPLATEFLOW_HOME is set up):

singularity run --cleanenv \
    -B ${PWD} \
    -B /path/to/templateflow_home:/SGLR/TEMPLATEFLOW_HOME \
    -B /path/to/license.txt:/SGLR/FREESURFER_HOME/license.txt \
    --env TEMPLATEFLOW_HOME=/SGLR/TEMPLATEFLOW_HOME \
    containers/.datalad/environments/fmriprep-20-2-3/image \
    ...
    --fs-license-file /SGLR/FREESURFER_HOME/license.txt \

For details, please refer to updated docs: https://pennlinc-babs.readthedocs.io/en/latest/preparation_config_yaml_file.html