Merge pull request #5865 from QiaoLei-88/make_icc_15_happy

avoid icc 15 error on non-const reference

diff --cc include/deal.II/base/tensor.h
index 7460ca3803d8f1b3fb6f8c60ec032b34974f84f0,e3bc3d3bc10ed601446b0c0f2326338c46ea5daf..26e6656b85d29d9194ab4d22e29df5994c974db8
--- 1/include/deal.II/base/tensor.h
--- 2/include/deal.II/base/tensor.h
+++ b/include/deal.II/base/tensor.h
@@@ -203,8 -203,13 +203,13 @@@ public
     * Number.
     */
    template <typename OtherNumber>
 -  Tensor<0,dim,Number> &operator = (const Tensor<0,dim,OtherNumber> &rhs);
 +  Tensor &operator = (const Tensor<0,dim,OtherNumber> &rhs);
  
+   /**
+    * Assignment from tensors with same underlying scalar type.
+    */
+   Tensor &operator = (const Tensor<0,dim,Number> &rhs);
+ 
    /**
     * This operator assigns a scalar to a tensor. This obviously requires
     * that the @p OtherNumber type is convertible to @p Number.