DPL: provide cast to string_view for header::Descriptor<16>

This will make some code compile under C++20, since C++20 is more aggressive in
doing implicit conversions where it currently uses the comparison operator.

DataFormats/Headers/include/Headers/DataHeader.h

@@ -224,14 +224,21 @@ struct Descriptor {
 
   // Note: don't need to define operator=(ItgType v) because the compiler
   // can use Descriptor(ItgType initializer) for conversion
+  using ImplicitConversion = std::conditional_t<(size <= 8), ItgType, std::string_view>;
 
   // type cast operator for simplified usage of the descriptor's integer member
-  // TODO: this is sort of a hack, takes the first element.
-  //       we should rethink these implicit conversions
-  operator ItgType() const
+  // in case it does not fit into the descriptor, the string representation is returned
+  operator ImplicitConversion() const
   {
-    static_assert(arraySize == 1, "casting Descriptor to ItgType only allowed for N<=8");
-    return itg[0];
+    if constexpr (std::is_same_v<ImplicitConversion, ItgType>) {
+      return itg[0];
+    } else {
+      size_t len = size;
+      while (len > 1 && str[len - 1] == 0) {
+        --len;
+      }
+      return std::string_view(str, len);
+    }
   }
 
   /// constructor from a compile-time string