stage4 update description and confirm correctness of proposed solution

[RedJocker]

there are two things not clear on description that can lead to miss-implementations. One is if brightness average should be calculated pixel by pixel or for the whole image (proposed solution adopts whole image) Second if brightness average(which is used to on contrast filter) should be calculated before applying brightness filter changes or after (proposed solution adopts before)

Also verify correctness of proposed solution on second item

[RedJocker]

tests updated to expect brightness average to be calculated after applying brightness filter on branch fixTests

todo: description update proposed solution update

[RedJocker]

description updated

todo: proposed solution update

[RedJocker]

new proposed solution is on alternativeSolution branch