This low-level function is
the analogue of tileindex for linear networks.
For a tessellation Z on a linear network,
and a list of query points on the same network,
the function determines which tile of the tessellation
contains each query point.
Argument Z should be a tessellation on a linear network
(object of class "lintess").
The vectors seg and tp specify the locations
of the query points, on the same network, using local coordinates:
seg contains integer values specifying which
segment of the network contains each query point;
tp contains numeric values between 0 and 1 specifying the
fractional position along that segment.
The result is a factor, of the same length as seg and tp,
indicating which tile contains each point. The levels of the factor
are the names of the tiles of Z.