ScanToTrack1D:

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   Initializes a new track. 

   d includes a field with a velocity estimate. The measurement
   directly initializes the position state.
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   Form:
   trk = ScanToTrack1D( xMeas, d, scan0, treeID, tag )
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   ------
   Inputs
   ------
   xMeas     (1,1)   Scan which is a position measurement
   d         (1,1)   Data structure
                       .v (1,1) Velocity
                       .a (3,3) State transition matrix
                       .b (3,1) Input matrix
                       .h (1,3) Measurement matrix
                       .p (3,3) State covariance matrix
                       .q (3,3) Model covariance matrix
                       .r (1,1) Measurement covariance matrix
   scan0     (1,1)   Scan number at which this track is created
   treeID    (1,1)   Track-tree ID. The track is inside this tree.
   tag       (1,1)   Unique tag to distinguish from all other tracks

   -------
   Outputs
   -------
   trk      (1,1) Track data structure

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Children:

MHT: TrackInitialization/MHTInitializeTrk