Add Tf2 versions of lookupTransform, lookupVelocity, setTransform

[kyle-basis]

This PR implements #726 - see there for initial discussion.

Some notes:

• No response was given for naming, so I went with *Tf2.
• I'm not totally pleased with lookupVelocity - tf2 doesn't have a dual vector type that I can find, and I didn't really want to implement a new Velocity for this PR. I'm happy to back that part of the change out - the function can be moved wholesale into tf2_ros when the time comes, instead, as it's not part of BufferCoreInterface.
• I combined the two manual reimplementations of toMsg into one reimplementation - it really shows that the interface is wrong here in the first place - the helper methods that should be used would cause a depedency loop.
• tf2::Transform doesn't have a child_frame_id - I think I did the correct thing, but it might be worth introducing somehow.
• ~~I'm unable to actually run the tests - I get an error on colcon build~~
• My version of colcon doesn't appear to have filter support, testing is a little hard. I think the solution here is to use an underlay, will experiment

[kyle-basis]

Only test fails are due to linting - do y'all have a script to fix this automatically?

[clalancette]

Only test fails are due to linting - do y'all have a script to fix this automatically?

I think you can run ament_uncrustify --reformat to fix it.

[kyle-basis]

Linting fixed - not sure why tf2_rospy is failing on jenkins, though

[MichaelOrlov]

@tfoote do you mind to take a look ?

@tfoote Friendly ping for review.