Vector operators

This MR extends the HOG to enable the generation of vector-vector operators using the tensorial vector function spaces.

Those are relevant for two reasons:

• communication patterns should improve compared to "block-scalar" operators
• enables the implementation of certain forms that are not possible or at least complicated with scalar operators (e.g., Nitsche + free-slip BCs)

Still WIP until the following points are resolved:

• P1 support (first 2D results are looking good)
• P2 support
• 3D
• automated testing
• viscous ops with blending should be tested to make sure the full feature set is covered
• cleanup (check duplicated code in hyteg-coupling, forms, ...)