Update testing dataset for new loader

[mariehbourget]

To test the new loader, we want to add those modalities to the data-testing dataset:

• microscopy PNG
• microscopy OMETIFF
• anat (2D single-slice nifti)
• dwi (4D nifti)
• eeg (files TBD)
• sessions.tsv architectures and files
• scans.tsv architectures and files

[mariehbourget]

microscopy PNG, sessions.tsv and scans.tsv are added in PR #6.

[mariehbourget]

Updates to do to the datasets following new loader first integration (ivadomed/ivadomed#699) for test_loader.py:

• add participant_id column in df_ref.csv
• add participant_id and sample_id in microscopy_png/df_ref.csv
• a df_ref.csv for multiple folders in path_data
• update df_ref.csv for test when roi_suffix is not None
• add CT-scan data