Add a node parameter to select header.stamp source and computation method

[rsiryani]

In the current implementation header.stamp is based on message processing ROS time unless a valid UTC time message is received.
We should simplify this and let the user choose which time reference he would like.

We should add a node parameter to either select:

• ROS only processing time (ros)
• INS absolute time referenced to UNIX epoch (ins_unix)
• INS absolute time referenced to GPS (ins_gps) - later
• INS absolute time referenced to UTC (ins_utc) - later

If we don't have yet a valid GPS/UTC time we should fallback to ROS system time.

ROS time is a large topic and any feedback from the community would be appreciated.