Enhancement - Mod/Att interaction

- New unit tests for Mod object.
- Added new constructors to Mod object for ease of use.
- Added Mod handling in Att object.
- Added more Att unit tests.

src/Att.cpp

@@ -126,9 +126,9 @@ std::string Att::composeF4 () const
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-void Att::addMod (const std::string&)
+void Att::addMod (const Mod& mod)
 {
-  throw std::string ("unimplemented Att::addMod");
+  mMods.push_back (mod);
 }
 
 ////////////////////////////////////////////////////////////////////////////////