Canonicalizes an XX/YY/ZZ interaction by swap/negate/shift-ing axes.

x The strength of the XX interaction.
y The strength of the YY interaction.
z The strength of the ZZ interaction.
atol How close x2 must be to π/4 to guarantee z2 >= 0

The canonicalized decomposition, with vector coefficients (x2, y2, z2)
satisfying 0 ≤ abs(z2) ≤ y2 ≤ x2 ≤ π/4 if x2 = π/4, z2 >= 0

Guarantees that the implied output matrix:

g · (a1 ⊗ a0) · exp(i·(x2·XX + y2·YY + z2·ZZ)) · (b1 ⊗ b0)

is approximately equal to the implied input matrix:

exp(i·(x·XX + y·YY + z·ZZ))