Fix artillery fire mission spread behaviour

[mharis001]

When merged this pull request will:

• title, Fix Artillery spread works more like an offset #99
• Fire each round individually at a random position around the center
• Rounds are not fired as fast but produces the correct behaviour that "spread" implies

[CreepPork]

Tested it, works like a charm:

• 0m spread
• 150m spread

But there's again a catch, the AI seem to do a little dance on each round that is fired.

[mharis001]

Yeah, that is the result of having to use doArtilleryFire with one round at a time, I don't think there is another way.

Could just use a single doArtilleryFire when spread is low (<10) since that should produce the expected result in that situation as well. Will help avoid this when using a small spread value, or none.

[Kexanone]

Yeah, when no spread is selected, just use doArtilleryFire once.