Stricter Mat3 -> Float64 criterion

Only convert an exchange matrix J to a Float64 if it is _exactly_
diagonal.

src/System/PairExchange.jl

@@ -14,9 +14,14 @@ function bond_parity(bond)
     return bond_delta > (0, 0, 0, 0)
 end
 
-# Convert J to Union{Float64, Mat3}
+# Convert J to Union{Float64, Mat3}. If J is _exactly_ the identity matrix, we
+# can compactly represent it using a single float. If J is near (but not
+# exactly) the identity matrix, retain the full matrix representation. This
+# could hypothetically be important to preserve symmetry breaking effects. For
+# example, a user might select J=diagm([a,a,a+ϵ]) for infinitesimal ϵ to favor
+# the z direction.
 function to_float_or_mat3(J)
-    if J isa Number || isapprox(J, J[1] * I, atol=1e-12)
+    if J isa Number || J == J[1] * I
         J = Float64(first(J))
     else
         J = Mat3(J)