Add a second constructor to avoid comparing size_t (unsigned int) and int

[esteve]

The comparison between thread_count (size_t) and -1 is causing issues when building transport-drivers as part of Autoware.Auto:

--- stderr: velodyne_nodes                                                                                                                                                              
In file included from /home/esteve/AutowareAuto/install/udp_driver/include/udp_driver/udp_driver.hpp:24,
                 from /home/esteve/AutowareAuto/src/drivers/velodyne_nodes/include/velodyne_nodes/velodyne_cloud_node.hpp:31,
                 from /home/esteve/AutowareAuto/src/drivers/velodyne_nodes/src/velodyne_cloud_node.cpp:25:
/home/esteve/AutowareAuto/install/io_context/include/io_context/io_context.hpp:62:45: error: unsigned conversion from ‘int’ to ‘size_t’ {aka ‘long unsigned int’} changes value from ‘-1’ to ‘18446744073709551615’ [-Werror=sign-conversion]
   62 |   explicit IoContext(size_t threads_count = -1);
      |                                             ^~
/home/esteve/AutowareAuto/src/drivers/velodyne_nodes/src/velodyne_cloud_node.cpp: In instantiation of ‘autoware::drivers::velodyne_nodes::VelodyneCloudNode<SensorData>::VelodyneCloudNode(const string&, const rclcpp::NodeOptions&) [with SensorData = autoware::drivers::velodyne_driver::VLP16Data; std::string = std::__cxx11::basic_string<char>]’:
/home/esteve/AutowareAuto/src/drivers/velodyne_nodes/src/velodyne_cloud_node.cpp:167:16:   required from here
/home/esteve/AutowareAuto/install/io_context/include/io_context/io_context.hpp:62:45: error: unsigned conversion from ‘int’ to ‘size_t’ {aka ‘long unsigned int’} changes value from ‘-1’ to ‘18446744073709551615’ [-Werror=sign-conversion]
/home/esteve/AutowareAuto/src/drivers/velodyne_nodes/src/velodyne_cloud_node.cpp: In instantiation of ‘autoware::drivers::velodyne_nodes::VelodyneCloudNode<SensorData>::VelodyneCloudNode(const string&, const rclcpp::NodeOptions&) [with SensorData = autoware::drivers::velodyne_driver::VLP32CData; std::string = std::__cxx11::basic_string<char>]’:
/home/esteve/AutowareAuto/src/drivers/velodyne_nodes/src/velodyne_cloud_node.cpp:168:16:   required from here
/home/esteve/AutowareAuto/install/io_context/include/io_context/io_context.hpp:62:45: error: unsigned conversion from ‘int’ to ‘size_t’ {aka ‘long unsigned int’} changes value from ‘-1’ to ‘18446744073709551615’ [-Werror=sign-conversion]
/home/esteve/AutowareAuto/src/drivers/velodyne_nodes/src/velodyne_cloud_node.cpp: In instantiation of ‘autoware::drivers::velodyne_nodes::VelodyneCloudNode<SensorData>::VelodyneCloudNode(const string&, const rclcpp::NodeOptions&) [with SensorData = autoware::drivers::velodyne_driver::VLS128Data; std::string = std::__cxx11::basic_string<char>]’:
/home/esteve/AutowareAuto/src/drivers/velodyne_nodes/src/velodyne_cloud_node.cpp:169:16:   required from here
/home/esteve/AutowareAuto/install/io_context/include/io_context/io_context.hpp:62:45: error: unsigned conversion from ‘int’ to ‘size_t’ {aka ‘long unsigned int’} changes value from ‘-1’ to ‘18446744073709551615’ [-Werror=sign-conversion]
cc1plus: all warnings being treated as errors
make[2]: *** [CMakeFiles/velodyne_cloud_node.dir/build.make:63: CMakeFiles/velodyne_cloud_node.dir/src/velodyne_cloud_node.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:188: CMakeFiles/velodyne_cloud_node.dir/all] Error 2
make: *** [Makefile:141: all] Error 2
---

[JWhitleyWork]

LGTM