Please add some clarifying info to MoveGroup tutorial

[mcevoyandy]

I find these lines extremely confusing. Could someone could unpack them into something more readable / understandable? Maybe the first is a really efficient way to do it, but it doesn't seem appropriate for a tutorial. JointModelGroup and RobotModel aren't discussed at all so the tutorial also seems very incomplete.

offending lines:
https://github.com/ros-planning/moveit_tutorials/blob/kinetic-devel/doc/pr2_tutorials/planning/src/move_group_interface_tutorial.cpp#L145

https://github.com/ros-planning/moveit_tutorials/blob/kinetic-devel/doc/pr2_tutorials/planning/src/move_group_interface_tutorial.cpp#L187

This portion could also use some clarification:
https://github.com/ros-planning/moveit_tutorials/blob/indigo-devel/doc/pr2_tutorials/planning/src/move_group_interface_tutorial.cpp#L167

I had thought that z_axis_tolerance = 3.14159 would mean that I just want the joint parallel to the floor, but making this change causes the solution to fail every time. What do these tolerances actually do and how would you use them to do something like "keep joint parallel to floor?"

[davetcoleman]

I can address this after #38 is merged

[davetcoleman]

#42

I've cleaned up that tutorial, though I'm not sure what to say about the axis constraints because I haven't used them.