Add tight trilinear form

[mewilhel]

• [X] 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)))))