Using concat method instead of append

[mirhnius]

Resolving issue #62

[mirhnius]

In the recent commit, I fixed a type incompatibility bug.

[jhlegarreta]

@mirhnius When git blaming eda4b21 I see that the output images have also changed:
https://github.com/carpentries-incubator/SDC-BIDS-sMRI/blame/eda4b219117046b4152c5f865649402abcd156f1/code/1_sMRI_modalities.ipynb

If it is just a deprecation error, then I assume the result of both commands should be the same. Can you please explain why the output images are changed?

When git blaming 457e4d0 it can be seen that the output images prior to the int64 to int32 change:
https://github.com/carpentries-incubator/SDC-BIDS-sMRI/blame/457e4d00554d339070720129f0593396d1aacd7e/code/4_sMRI_segmentation_parcellation.ipynb#L1204

are also changed, e.g.
https://github.com/carpentries-incubator/SDC-BIDS-sMRI/blame/457e4d00554d339070720129f0593396d1aacd7e/code/4_sMRI_segmentation_parcellation.ipynb#L549

Why is this?