Resolve "Bug in modem sim node. Does not publish sim range for every packet"

Bugfixes

• bug fix in modem_sim_node. When calculating the OWTT for every sim packet, we were subtracting two different time types (rospy.Time and ros_acomms_msgs.Time). We are now using transmission_delay directly. This is type rospy.Time which has the to_sec() method.

Closes #55 (closed)