[ENH] Derive masks to support native-space data

[tsalo]

References #6.

Changes proposed in this pull request:

• Add function to derive masks from EPI data and CSF tissue probability map or mask.
• Have the ICA function return the relevant ICA outputs instead of inferring the filenames in the main workflow function.
• Use the new derived masks throughout the workflow.

[tsalo]

Closing now that https://github.com/Brainhack-Donostia/aroma/pull/24 is open.