ENH: add 2D det() / inv() methods for Tensor/SymmTensor (#2724)

- for cases where a 3D tensor is being used to represent 2D content,
  the determinant is zero. Can use inv2D(excludeDirection) to compensate
  and invert as if it were only 2D.

ENH: consistent definitions for magSqr of symmTensors, diagSqr() norm

COMP: return scalar not component type for magSqr

- had inconsistent definitions with SymmTensor returning the component
  type and Tensor returning scalar. Only evident with complex.

applications/test/SymmTensor/Test-SymmTensor.C

@@ -370,7 +370,7 @@ void test_global_funcs(Type)
                                 Type(126)
         )
     );
-    cmp("  Square of Frobenius norm = ", magSqr(sT), Type(205));
+    cmp("  Square of Frobenius norm = ", magSqr(sT), scalar(205));
 }