Afni parser

[hermann74]

• begin for parsing afni file
• start with example from nidm : https://github.com/incf-nidash/nidmresults-examples/blob/master/afni_default/subject_results/group.DS0011/subj.sub_001/proc.sub_001
• adapt code from fsl parser (with a .sjson describing signature of afni's command) for afni parser
• filtrer from input bash file :
  • foreach loop
  • if conditop
  • # comments
  • set and echo functions
• make unit test for afni readlines subfunction
• Into github actions, add fetching nidm afni example, parsing to .json , and visualize as png.

[cmaumet]

Hi again @hermann74 @cyril-data! I've just realised that this PR also removes the original examples we had built by hand examples/fsl_default and examples/spm_default. It would be great if we can keep them (until we are sure everything we had in there is implemented in the parser). Can you add those back?