`Mineral.update_orientations` API should accept time-dependent velocity gradient callables

[adigitoleo]

At the moment, for the third arg it accepts a callable with signature L(t) but really it should be accepting L(t, x) since the velocity gradient L can change with both position and time.

Places where the API needs to be fixed:

• [x] velocity.py
• [x] test_simple_shear_2d.py
• [x] test_vortex_2d.py
• [x] test_corner_flow_2d.py
• [x] test_simple_shear_3d.py
• [x] minerals.py
• [x] pathlines.py
• [x] visualisation.py
• [x] Fluidity advection example
• [x] Fluidity cornerflow example (#179)

[adigitoleo]

Also change every velocity callable to use u(t, x) as well.