[!576] [math] deduce the return value of Power in order to allow unit value...

[!576] [math] deduce the return value of Power in order to allow unit value types (see dune-grid#79)

Merge branch 'power-auto' into 'master'

ref:core/dune-common When working with unit value types (e.g. meters, seconds,
\...) the return type of Power is not the value type. This MR changes the
return type to auto to enable automatic type deduction.

See merge request [!576]

  [!576]: gitlab.dune-project.org/core/dune-common/merge_requests/576

dune/common/power.hh

@@ -46,7 +46,7 @@ namespace Dune {
   struct Power
   {
     template <typename T>
-    static constexpr T eval(const T & a)
+    static constexpr auto eval(const T & a)
     {
       return PowerImp<p>::eval(a);
     }
@@ -57,9 +57,9 @@ namespace Dune {
   struct PowerImp<p,false>
   {
     template <typename T>
-    static constexpr T eval(const T & a)
+    static constexpr auto eval(const T & a)
     {
-      T t = Power<p/2>::eval(a);
+      auto t = Power<p/2>::eval(a);
       return t*t;
     }
   };
@@ -68,7 +68,7 @@ namespace Dune {
   struct PowerImp<p,true>
   {
     template <typename T>
-    static constexpr T eval(const T & a)
+    static constexpr auto eval(const T & a)
     {
       return a*Power<p-1>::eval(a);;
     }
@@ -78,7 +78,7 @@ namespace Dune {
   struct PowerImp<1,true>
   {
     template <typename T>
-    static constexpr T eval(const T & a)
+    static constexpr auto eval(const T & a)
     {
       return a;
     }