This output-oriented mapping
requires that the
data structure that describes the geometry
of the whole output
(`out_geom`)
is passed to `genkir3d`.
The access to the output data can be blocked along
all the output space dimensions;
that is the midpoint axes and the offset and azimuth axes.
The geometry of the blocks is specified
with the argument `out_geom_block`.
To localize the access to the mapping tables,
the inner loops over the output space are
in the midpoint coordinates (`cmp_x` and `cmp_y`)
of `out_geom_block`.

A Sep3d data set can be either gridded or ungridded.
An ungridded data set is accessed sequentially.
The data are read multiple times,
as many times as the number of output blocks.
A gridded data set is accessed selectively
according to the offset and the azimuth of the
output block being currently built.
Furthermore, for efficiency purposes,
the maximum aperture of the operator along the midpoint dimensions
can be specified in the `param` data structure,
limiting the ranges of midpoints of the input data
being accessed for each output block.

7/5/1998