Merge: 3 activities FSL data model including varcope and zstat

This is a proposal to update the FSL NI- data model by:
- Creating separate `Model fitting` and `Contrast estimation` activities to comply with recent changes made on SPM example.
- Creating `Contrast variance map` and `Z-statistical Map` as new FSL-specific entities.

This PR also includes terms updates as proposed in PR #39. (PR #39 should be reviewed first)