[REF] Remove FSL from dependencies

[tsalo]

Closes #524. Related to PennLINC/xcpd_build#2. Also related to #338.

Changes proposed in this pull request

• Replace calls to fslmerge with nilearn.image.concat_imgs.
• Replace calls to fslsmooth with nilearn.image.smooth_img.
• Replace calls to fslmaths with nilearn.image.math_img.

Documentation that should be reviewed

None.

[kahinimehta]

@tsalo might need to update the smoothing tests to reflect the removal of FSL

[tsalo]

@kahinimehta done! Just waiting for the Docker image to deploy and then I'll update the Dockerfile.