More packed representation of the Wigner small d rotation matrix
This matrix has an additional symmetry which I didn't notice when I first wrote the code.
d^{j}_{m', m}(\beta) = -1^{m'-m} d^{j}_{m, m'}(\beta) = d^{j}_{-m, -m'}(\beta)
This allows omitting elements which can be obtained from the symmetry relation