mgi: add execution methods for moveit_msgs::RobotTrajectory

[v4hn]

I just reviewed moveit/moveit_tutorials#468 and
thought this would be a nicer way to execute Cartesian trajectories...

[rhaschke]

Instead of introducing two new function variants, what about introducing a single (implicit) constructor of Plan from RobotTrajectory? This way, it should be possible to pass a RobotTrajectory to the existing functions.

[v4hn]

Actually, if you want to unify them, I would propose to simply deprecate the old execute method.
The method actually only ever looks at the trajectory_ field...
I'll implement the changes and provide a deprecated wrapper for the old method.

[v4hn]

After thinking about it a bit more, I decided to leave the old interfaces around without deprecation.
They are convenient wrappers when working with the plan method and in use all over the place.
It feels stupid to deprecate a one-line convenient wrapper in a class meant to make access convenient.

[codecov-io]

Codecov Report

Merging #1955 into master will increase coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1955      +/-   ##
==========================================
+ Coverage   49.83%   49.87%   +0.04%     
==========================================
  Files         314      314              
  Lines       24720    24726       +6     
==========================================
+ Hits        12319    12332      +13     
+ Misses      12401    12394       -7

Impacted Files	Coverage Δ
...moveit/move_group_interface/move_group_interface.h	36.84% <ø> (ø)	⬆️
.../move_group_interface/src/move_group_interface.cpp	22.38% <0%> (-0.15%)	⬇️
...veit_experimental/moveit_jog_arm/src/jog_calcs.cpp	73.98% <0%> (+4.07%)	⬆️

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[felixvd]

It makes a lot of sense for the move group to execute a trajectory instead of a plan. I like this.