Log a warning when a @RootContext field isn't populated

Related to #674 

When a bean is injected in another class which has a different Context
from the required @RootContext field class, the field is left null. This
could produce hard to find bugs. The fix adds a log to warn the user.

The generated code from `EnhancedClass_` : 

```java
 @SuppressWarnings("all")
    private void init_() {
        if (context_ instanceof Activity) {
            Activity activity = ((Activity) context_);
        }
        OnViewChangedNotifier.registerOnViewChangedListener(this);
        Resources resources_ = context_.getResources();
        hello = resources_.getString(string.hello);
        activityManager = ((ActivityManager) context_.getSystemService(Context.ACTIVITY_SERVICE));
        customApplication = SampleRoboApplication_.getInstance();
        if (context_ instanceof ThreadActivity) {
            threadActivity = ((ThreadActivity) context_);
        } else {
            Log.w("EnhancedClass_", (("Due to Context class "+ context_.getClass().getSimpleName())+", the @RootContext ThreadActivity won't be populated"));
        }
        context = context_;
        if (context_ instanceof Service) {
            service = ((Service) context_);
        } else {
            Log.w("EnhancedClass_", (("Due to Context class "+ context_.getClass().getSimpleName())+", the @RootContext Service won't be populated"));
        }
        if (context_ instanceof Activity) {
            activity = ((Activity) context_);
        } else {
            Log.w("EnhancedClass_", (("Due to Context class "+ context_.getClass().getSimpleName())+", the @RootContext Activity won't be populated"));
        }
        secondDependency = SecondDependency_.getInstance_(context_);
        calledAfterInjection();
    }
```

I don't know very well Codemodel yet, please check if I used it well.