Add constants for tuple fields

Add constants for use with std::get.

Tested:  Run unit tests

Change-Id: Ic09c13feeda69d61c98f63d227cae8f08d1bf50e
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/src/journal.cpp b/src/journal.cpp
index 7069c74..f54c5b3 100644
--- a/src/journal.cpp
+++ b/src/journal.cpp
@@ -26,11 +26,11 @@
 {
     for (const auto& n : index)
     {
-        const auto& path = std::get<0>(n.first);
-        const auto& pathMeta = std::get<0>(n.second);
-        const auto& propertyMeta = std::get<1>(n.second);
-        const auto& storage = std::get<2>(n.second);
-        const auto& value = std::get<0>(storage.get());
+        const auto& path = std::get<pathIndex>(n.first);
+        const auto& pathMeta = std::get<pathMetaIndex>(n.second);
+        const auto& propertyMeta = std::get<propertyMetaIndex>(n.second);
+        const auto& storage = std::get<storageIndex>(n.second);
+        const auto& value = std::get<valueIndex>(storage.get());
 
         if (!value.empty())
         {