Gizmo mfv clean

See merge request !587

Add output_list test

See merge request !586

Fixed sign error in gravitational mass flux computation. GizmoMFV now conserves energy in the Evrard collapse test.

Changed some things in the Gizmo MFV implementation to improve the accuracy (also fixed the total energy mode).

Cleaned up some equations in Gizmo's hydro_iact and hydro_gradients. Moved the gradient prediction to before the velocity boost.

No unskip before rebuild

See merge request !585

Dump snapshots if the next time is strickly greater than the dump time.

Closes #447

See merge request !583

Do not unskip tasks if we already know we will have to rebuild. Minor message fixes in engine_maketasks().

Velociraptor fixes

See merge request !584

Fix output_list

See merge request !582

Use clang-format-5.0 instead of clang-format-3.8 for the code's style.

See merge request !580

Output time from a file

See merge request !560