// Do symmetric extension of data using prescribed symmetries
// Original values are in output[npad] through output[npad+size-1]
// New values will be placed in output[0] through output[npad] and in output[npad+size] through output[2*npad+size-1] (note: end values may not be filled in)
// extension at left bdry is ... 3 2 1 0 | 0 1 2 3 ...