Fix for ACTS-273 in Surface/DiamondBounds
Original code hid the baseclass protected data member and prevented even simple construction; this addresses that, corrects one typo and updates all 'at' vector accessors to simple bracket accessors.
Original code hid the baseclass protected data member and prevented even simple construction; this addresses that, corrects one typo and updates all 'at' vector accessors to simple bracket accessors.