Add tight trilinear form

[mewilhel]

• Add the basic trilinear relaxation
• Compute and plot relaxation cases as a sanity check.
• Check performance (no allocations in benchmarking)
• Add unit tests for trilinear
• Default to the trilinear relaxation for *(x,y,z)
• Add methods for *(x, y, z, a, b, c) and so on which use use the trilinear form rather than *(x, *(y, *(z, *(a, *(b, c)))))