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> >.
Merge pull request #123 from kronbichler/master

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