Added a service to set the speed slider position (#454)

Squashed commits:

* Added a service to set the speed slider position
* Use a more specific name for the data
  The name used is the same as defined in the RTDE interface.
* Added an explanation with valid data range to the service.
* Update comment.
* Renamed service to SetSpeedSliderFraction
  This better represents the technical background
* Use correct name of svc def file.
* Fixup

ur_msgs/CMakeLists.txt

@@ -23,6 +23,7 @@ add_message_files(
 add_service_files(
    FILES
    SetPayload.srv
+   SetSpeedSliderFraction.srv
    SetIO.srv
 )
 

ur_msgs/srv/SetSpeedSliderFraction.srv

+# Set the speed slider on the teach pendant to the specified value.
+#
+# Values for 'speed_slider_fraction' must be from [0; 1.0]; values outside this
+# valid range will result in an error being returned to the caller.
+
+float64 speed_slider_fraction
+---
+bool success