dwi2tensor: Fix weights recalculation

- Previously, if -iter 1 was specified, the weights were not recomputed, and so results were identical to that with -iter 0.
- Do not unnecessarily recompute weights at completion of the final iteration.

cmd/dwi2tensor.cpp

@@ -149,7 +149,7 @@ class Processor { MEMALIGN(Processor)
           work.setZero();
           work.selfadjointView<Eigen::Lower>().rankUpdate (b.transpose()*w.asDiagonal());
           p = llt.compute (work.selfadjointView<Eigen::Lower>()).solve(b.transpose()*w.asDiagonal()*w.asDiagonal()*dwi);
-          if (maxit > 1)
+          if (it < maxit)
             w = (b*p).array().exp();
         }