Powder temperature effect

NEEDS CHECKING BEFORE MERGING!

As it was discussed here #995. PT has large influence on initial speed of bullet.

initial speed for diffrent powder Temperatures:
Temp(K)//init vel //Temp_change vs velocity_change			
268//884,52//2,48
273//890,9//2,48
278//897,26//2,48
283//903,63//2,48
288//910//0
293//916,37//2,48
298//922,74//2,48
303//929,11//2,48
*http://ballistics.eu/exterior.html
*standard temperature set as 288 K based on International Standard Atmosphere

Acording to table above this influence is linear and looks like this:
_velocity_update = _velocity*((AGM_Wind_currectTemperature/288-1)/2.5+1 )
AGM_Wind_currectTemperature variable doesn't exist yet but was created here: #1039

Someone please check file I'm not  any good in scripting