Fix incorrect registration of behavior trees containing faulty XML

[schornakj]

This fixes a problem where a behavior tree containing an invalid structure (for example, a BehaviorTree with less than one child or more than two children) could still be instantiated by the factory, even though the XML parser threw and exception when trying to load it. This was possible because adding a BehaviorTree's name to the list of available behavior trees occurred before the check for the validity of the BehaviorTree's XML.

• Modify XML parsing to add a BehaviorTree node to the list of available trees only if that BehaviorTree is valid.
• Delete unused tree_names and tree_count variables.
• Extend the unit tests to reflect this error case. These tests fail without the fix in this PR.
• Move XML parsing helper functions into an anonymous namespace so they can be used in more places.

[facontidavide]

Thanks for the help.
But I think that you should not remove the function from having public access, since Groot 1.0 relies on that.

Also, seems like you moved some code without much modification, and this makes it very hard for me to understand if you changed something or not.