90mm gun does no damage against gbrAECII

I noticed that the itasemovente90 does no damage against british AEC at front. By giving the weapon impactonly=1; the semovente do damage against the AEC from the front. Looking at "weapons/Armour/MediumHeavyGun.lua" those do also have impactonly=1;
We could also consider fixing the AEC so that it does take damage even if a weapon hits it in the front, have small areaOfEffect and have impactonly=0.
Merge pull request #2 from Godde/patch-1

90mm gun does no damage against gbrAECII