Replace argument replacements with properties

[urfeex]

Otherwise that won't work when including this from another xacro where those arguments aren't available.

Consider this simplified URDF:

<?xml version="1.0"?>
<robot xmlns:xacro="http://wiki.ros.org/xacro" name="ur">
   <!-- import main macro -->
   <xacro:include filename="$(find ur_description)/urdf/ur_macro.xacro"/>

   <xacro:arg name="ur_type" default="ur5e"/>

   <xacro:arg name="tf_prefix" default="" />
   <xacro:arg name="joint_limit_params" default="$(find ur_description)/config/$(arg ur_type)/joint_limits.yaml"/>
   <xacro:arg name="kinematics_params" default="$(find ur_description)/config/$(arg ur_type)/default_kinematics.yaml"/>
   <xacro:arg name="physical_params" default="$(find ur_description)/config/$(arg ur_type)/physical_parameters.yaml"/>
   <xacro:arg name="visual_params" default="$(find ur_description)/config/$(arg ur_type)/visual_parameters.yaml"/>

   <xacro:arg name="initial_positions_file" default="$(find ur_description)/config/initial_positions.yaml"/>

   <xacro:property name="initial_positions_file" default="$(arg initial_positions_file)"/>

   <link name="world" />

   <xacro:ur_robot
     name="ur5e"
     tf_prefix=""
     parent="world"
     joint_limits_parameters_file="$(arg joint_limit_params)"
     kinematics_parameters_file="$(arg kinematics_params)"
     physical_parameters_file="$(arg physical_params)"
     visual_parameters_file="$(arg visual_params)"
     sim_gazebo="false"
     sim_ignition="true"
     >
     <origin xyz="0 0 0" rpy="0 0 0" />
   </xacro:ur_robot>
</robot>

The sim_ignition parameter is passed to the ur_robot_macro, as one might expect it. However, with the humble branch version this will fail with

$ xacro test.urdf
Undefined substitution argument sim_ignition
when instantiating macro: ur_ros2_control (/home/robot/checkout/ur_humble/colcon_ws/install/ur_description/share/ur_description/urdf/ur.ros2_control.xacro)
instantiated from: ur_robot (/home/robot/checkout/ur_humble/colcon_ws/install/ur_description/share/ur_description/urdf/ur_macro.xacro)
in file: test.urdf

Inside the macro was read from an argument, that is defined outside of the macro file. This only coincidently works as long as we use it in a xacro structure where this argument is defined.

This PR reads the value not from the global argument but from the macro parameter which gets stored into a property.

The test URDF from above works fine with the changes from this branch.