Compare ASLPrep and niworkflows skullstrip workflows

[tsalo]

Summary

Need to document differences between aslprep.workflows.asl.util.init_enhance_and_skullstrip_asl_wf and niworkflows.func.util.init_enhance_and_skullstrip_bold_wf.

[tsalo]

Here are notes from #299:

• There was an image dilation step applied to the input image, then a header-check to make sure the dilation step didn't mess up the header.
• There was a pre_mask bool parameter for when the mask was vs. was not already calculated. The "mask was not pre-generated" logic has been removed, even though the parameter remains.
• However, in the current version of sdcflows, init_enhance_and_skullstrip_bold_wf is only used for the PEPOLAR workflow, which shouldn't apply to ASL data. I think this means that we can switch to the newer version of sdcflows and not need the modified version of init_enhance_and_skullstrip_bold_wf we currently have.