Avoid setting the templated 'Number' to '0' (double) in trace(Tensor<2,dim,Number>)

Enables using trace also with e.g. `Tensor<2,dim,VectorizedArray<double> >.`
I wanted to test trace on Tensor<2,dim,VectorizedArray<double> > which was the original motivation for the patch. I realize that I did not change the test thoroughly enough, which should now be addressed (also test both trace(Tensor) and trace(SymmetricTensor)).