[FT] AO minor problem

If you leave the tow area blank instead of having a 0 there the results get a
bit odd.

Mass 100 ship, ave hull, thrust 4, ftl

40 mass free if tow block is 0 49 mass free if thrust block is blank

drives for small amounts of tow mass appear to be wrong.

100 mass ship, add a tow mass of 2 and drive mass does not increase, this
should probably be a minimum of 1.

> If you leave the tow area blank instead of having a 0 there the

and

> drives for small amounts of tow mass appear to be wrong.

Thanks for the bug reports. I'll check it out.

- Sam