Rounding of native function math.muldiv should be documented

Created by: stephanemagnenat

Native function math.muldiv may produce rounding errors, for instance 2/3 is 0 rather than 1. It might be interesting for most end-user applications for math.muldiv to actually round correctly the numbers. To resolve this problem, one should:

• Write unit tests exhibiting the problem.
• Implement rounding (https://github.com/aseba-community/aseba/blob/master/vm/natives.c#L821), by testing the sign and pre-adding half of the divisor to the dividend.