struct PK_LINE_sf_s
    {
    PK_AXIS1_sf_t basis_set; --- a cartesian point and a direction
    };

typedef struct PK_LINE_sf_s PK_LINE_sf_t;




This is the standard form for an unbounded straight line which passes through
the given location in the given direction.

The parameterisation of the line is as follows:

    P=basis_set.location
    V=basis_set.direction

    P( t ) = P + t*V