Add support for time invariant 'delta' filter

[chadrockey]

The current velocity filter is dependent on the timestamps for both the internals of the filter ("calculating velocity from position/orientation updates") and for the output ("using system time"). This causes some instability in the filter when playing back from bags due to the relatively low time dt.

Some users never factor time into their odometry, so having a filter calculate dx_robot, dy_robot, and dheading_robot should produce more stable results.

Not yet implemented because I need to decide if this should be the default for non-global corrections.