Fixed Odometry step_time calculation

[JustSomeRandomUsername]

The calculation for the step_time in the Odometry.cpp was comparing builtin_interfaces/msg/Time nanosec the nanosecond component of that message to rclcpp::Time nanoseconds() which is a absolute measure of time in nanoseconds. This lead to step_time being negativ, in tern flipping the sign of the resulting odometry.

Additionally the last_time variable was set to msg time before calculation, not after.

I created a private uint32 last_time_ in Odometry.hpp, and switched the calculations to use the msg/Time nanosec value. This will result in bad behaviour if the time difference is larger than 1 sec. But for my use case that is not a Problem