Move MRI anat files into a specific BIDS dataset folder

As discussed previously in PR #6, we should have different BIDS dataset folders inside the data-testing repo for each modality we want to test. We would like to eventually add other datasets such as microscopy in ome-tiff file format, eeg and dwi.

Right now, the microscopy dataset in png file format in under the folder `microscopy_png`, but the `mri anat` dataset is at the root of the repo.
We should re-organize the `mri anat` files into a specific folder for this dataset and update the tests in ivadomed accordingly.

@ahill187, would you like to look into that?