Create spectral mass operator with GLL points and quadrature

 - Precompute detJ (send to device)
 - Reorder dofmap (and send to device)
 - Apply gather (https://github.com/Excalibur-SLE/wave-fenics/pulls)
 - Apply transform (https://github.com/Excalibur-SLE/wave-fenics/tree/igor/spectral-mass)
 - Apply scatter (https://github.com/Excalibur-SLE/wave-fenics/pulls)

No GEMM required. This operator is memory bound by definition.