[osg-submissions] try doubles for math inLineSegment::intersect() methods

[Robert Osfield]

The first step towards Vec3d support in LineSegment I have introduced
a typedef into LineSegment:

        typedef Vec3d vec_type;
        typedef vec_type::value_type value_type;

I have also changed various methods and intenals of LineSegment to use
these typedefs rather than just Vec3.  Its a first step as I haven't
changed all the internals to use floats - they should now use the
value_type method.  We should also have several intersect functions,
ones with float/Vec3f and ones with double/Vec3d to ensure that
calling code can remain using the types that suit the calling code.

I have checked these changes as they stand to get the ball rolling.
Andy's changes need to be reviewed against mine, and a new set of
changes made to complete the work above.

Robert.