BEFORE THIS COMMIT / How to reproduce :

Consider a multiple choice question with single answer.

A. 2 points
B. 1 point
C. -1 point

Here, the user can only select one answer. He picks A. He only obtains 2/3 points.
This is because the maximum score was computed as the sum of all positive scores
for the question. It does not make sense, as the student can only pick one answer.
Therefore, the formula does not allow giving partial scores (here, answer B, close
to the correct answer) while maintaining correct score percentages.

AFTER THIS COMMIT:

The sum of positive scores is still used as maximum score for multiple choice
with multiple answers, as we want the student to pick all good answers for max score.

However, for multiple choice with single answer, we use the maximum positive score
(or 0, if none) among answers, as it the maximum score one can get while selecting
a single answer.

-> example above : I pick A, I obtain 2 points out of max(2, 1, (0)) = 2
-> I obtain max score for the question. 2/2.

ps: As the sum only uses striclty positive values, the negative points are still
operational and the flow is not changed.

TESTS:

A test test_partial_scores_simple_choice is added to make sure the simple_choice
is repaired and always gives 100% of points if the best answer is picked.

Task-2533836

closes odoo/odoo#76326

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>