Migrate rtt_tf to tf2

[francisco-miguel-almeida]

This branch ports rtt_tf to be now based on tf2. It also allows subscribing to static transforms as input.
As requested in issue #68.

[meyerj]

Looks good. I only have a few comments and requests:

• Static transforms have to be added as such to tf2::BufferCore::setTransform().
• I suggest to add support for broadcasting static transforms, too.
• Remove commented/obsolete code

Not covered by comments below:

• rtt_tf needs to depend on the new typekit package rtt_tf2_msgs. Include the generated header rtt_tf2_msgs/typekit/Types.hpp instead of tf2_msgs/TFMessage.h, which declares the template instantiations for RTT::InputPort<tf2_msgs::TFMessage> and RTT::OutputPort<tf2_msgs::TFMessage> as external. This pattern should always be used for types that have a typekit to reduce binary sizes and save compilation time.
• Update rtt_tf/package.xml and rtt_tf/CMakeLists.txt. At the moment it only works because tf depends on tf2_ros internally. We can keep the dependency to tf and still use tf::resolve() instead of copying the method.

When adding new or updating operations of the component, please do not forget to also update the public tf_interface.h, too.

[francisco-miguel-almeida]

Addressed most issues, but as we discussed earlier, some bigger changes should be addressed as a separate PR. See issue created to discuss this: #126