TrigNavTools: Move getFeatureOccurrence() out of TrigNavigationSlimmingTool.
Move getFeatureOccurrence() out of TrigNavigationSlimmingTool, making it a free function. Do the same with getLabel(). In preparation for merging TrigNavigationSlimmingTool into TrigNavigationThinningSvc.
Adjust friendship in TrigNavigation.
Use free function in TrigValAlgs, and drop dependency on TrigNavigationSlimmingTool.