Explosion code refactor

[Coolthulhu]

Changed the PR to be more general:

• Moved the explosion code to new files: explosion.h and explosion.cpp
• Explosion actor and explosion in fire now use the same structure, which can be loaded from a json with a single function
• Added the ability of ammo to use its explosion data on ranged hit
• Explosion structure contains the shrapnel data
• Added game::explosion( tripoint, explosion_data ) - much cleaner than all those arguments. Changed the old explosion function to a be an overload of the 2 argument one.

[mugling]

Nice. I was considering looking into the 40mm ammo and this PR should presumably make that much easier?

[Mshock777]

So here's a little story. Back in the days when I was just starting to mess with Cata I made a PR #8727 where I rebalanced some explosives due to the fact that we don't have vehicle armour. If you are going to overhaul explosions you probably should look at how stats where before that PR.

[BevapDin]

Maybe it's time to move the explosion code into a separate place (namespace? or make it a member of explosion_data).

With the additional parameters and return type from @mugling's shrapnel PR, the explosion function should accept a single parameter that contains all the explosion data (for example explosion_data). That would make adding more data to it much easier.

[Coolthulhu]

the explosion function should accept a single parameter that contains all the explosion data (for example explosion_data).

Went this route.

explosion is still a game method, but now it can be invoked with just 2 arguments.