Casting

[tgustaf]

When using this code on my MTi-G-710, the code kept crashing with the messege
[xsens_mti_node-1] what(): cannot store a negative time point in rclcpp::Time
This seems to be due to a casting issue in timereferencepublisher.h, where
sec is declared to be an uint32_t, but in the rclcpp::Time sample_time() it is
expected to be an int32_t.