create a `cpp_spm` main function in the root folder

[Remi-Gau]

This should be able to control:

• initialization
• uninitialization
• run tests
• update
• help to provide a BIDS app like API
  • preprocess
  • stats

cpp_spm('analysis_level', 'participant', ...
        'participant_label', {'01', '02'}, ...
        'analysis_type', 'preproc', ...
        'input_dataset, input_path, ...
        'output_path', some_path, ...
        'bids_filter_file', 'bids_filter.json'
        'options', 'something.json'
        ...

optional:

• init and update submodules