Adjust fuel tanks rear channel flanges

The fuel tank rear channel flanges butt right against the main spar flanges, yet the mismatch was up to 1.5mm. Some adjustment of these flanges would be needed to avoid possible issue when closing up the tanks. A seaming tool made quick work of this. Perfection not needed here, just to reduce these larger discrepancies. Most of the adjustment was to the bottom edge of the channels. When doing this the channels were clecoed in place with the tank mount brackets