[New module] Freesurfer synthregistration

[anroy1]

Describe your changes

Initial module PR for registration tool using Freesurfer synthmorph

Say which test data are used by your module

Checklist before requesting a review

• Create the tool:
  • Edit ./modules/nf-scil/<category>/<tool>/main.nf
  • Edit ./modules/nf-scil/<category>/<tool>/meta.yml
• Generate the tests:
  • Edit ./tests/modules/nf-scil/<category>/<tool>/main.nf
  • Edit ./tests/modules/nf-scil/<category>/<tool>/nextflow.config
  • Add test data locally for tests with the fork repository
  • Generate the test infrastructure and md5sum for all outputs
• Ensure the syntax is correct :
  • Check indentation abides with the rest of the library (don't hesitate to correct others !)
  • Lint everything. Ensure your variables have good names.

[AlexVCaron]

Again, great work ! Syntax and wording, then merge is on !