struct PK_NABOX_sf_s
     {
     PK_AXIS2_sf_t basis_set;    --- coordinate system for box
     PK_BOX_t      box;          --- box
     };
typedef struct PK_NABOX_sf_s PK_NABOX_sf_t;


This data structure is the standard form for a non-aligned box


A non-aligned box is a cuboid space defined by an origin, a pair of orthogonal
axes (normalised direction vectors) and a set of x, y, z intervals. The term
`non-aligned' means the box axes need not be aligned with the model space axes.

The first axis is called `the reference direction' (the local X direction).

The second axis is simply called `the axis'; this is the local Z direction.

The local Y direction is formed from the cross product of the second and
first direction vectors          i.e.  local_Y  =  local_Z CROSS local_X
These form a right handed coordinate system.

                                y
                                |
                                |
                               / \
                              /   \
                     (axis) z/     \x (ref_direction)

The associated box is defined in this local coordinate system.
Its extent is defined by x, z and z intervals