Added THIS IS NOT RIGHT note

  if (model->nonlinearflag)

    descriptor->forward(map, list, descriptors);

  // ***********THIS IS NOT RIGHT**********************

  // This whole idea is flawed. The gamma matrix is too big to

  // store. Instead, we should generate the A matrix,

  // just as ComputeSNAP does, and then pass it to 

  // the model, which can evaluate gradients of E, F, sigma,

  // w.r.t. model parameters.

  // calculate descriptor contributions to parameter gradients

  // and gamma = double gradient w.r.t. parameters and descriptors