VAB: minor improvement of radial decoupler to tank attachment

[DBowman]

I think there is really only one angle one wants tanks to attach to a radial decoupler - 'square' (where the centre of the attached tank is on the line radially out from the centre of the coupler). Usually you have to futz about micro moving the mouse to get the tank to attach squarely, it's 'easy enough' to get it right, but it is a constant pointless anoyance. If there is a case for 'skew' attachment then the 'snap right' on off could be bound to the angle snap setting.