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/propertywatch.cpp b/src/propertywatch.cpp
index defde78..63a2c31 100644
--- a/src/propertywatch.cpp
+++ b/src/propertywatch.cpp
@@ -36,9 +36,9 @@
 
     for (const auto& i : index)
     {
-        const auto& path = std::get<0>(i.first);
-        const auto& interface = std::get<1>(i.first);
-        const auto& property = std::get<2>(i.first);
+        const auto& path = std::get<pathIndex>(i.first);
+        const auto& interface = std::get<interfaceIndex>(i.first);
+        const auto& property = std::get<propertyIndex>(i.first);
         m[path][interface].push_back(property);
     }