Add new controller_step RTDE field (UniversalRobots#514)

RTDE gets a new field called "controller_step", see
https://docs.universal-robots.com/tutorials/communication-protocol-tutorials/rtde-guide.html

This PR adds it to the list of possible data fields.

Author: urfeex

src/rtde/data_package.cpp

@@ -454,6 +454,7 @@ std::unordered_map<std::string, DataPackage::_rtde_type_variant> DataPackage::g_
   { "time_scale_source", int32_t() },
   { "target_gravity", vector3d_t() },
   { "target_base_acceleration", vector6d_t() },
+  { "controller_step", uint64_t() },
 
   // NOT IN OFFICIAL DOCS
   { "tool_digital_output_mask", uint8_t() },

tests/resources/exhaustive_rtde_output_recipe.txt

@@ -402,3 +402,4 @@ script_control_line
 time_scale_source
 target_gravity
 target_base_acceleration
+controller_step