[YaspGrid][Bugfix] Fix new outside entity movement code in intersection

The new entity movement code in 4b228a1 does not work correctly if both
movements happen to be in the same direction. In that case, the old code
overwrote the old movement instead of accumulating the two.
[YaspGrid] Remove unnecessary const_cast

I don't know why GCC originally wanted me to do a const_cast to call
update(), but it really isn't necessary.