unit config additions

[pvutov]

The following fields should be moved from hardcoded to being in the unit config:

shell explosion fx, shell explosion size
unit explosion size (maybe)
convert move speeds to a sane representation
'prefabAdditions' field that allows adding meshes to the main model, such as turrets Declined: If someone needs to do this, they will have to work with the prefab in Unity anyway. So it is easier to just have that person make a full prefab. This is a rare use case and if we were to support it it would mean extra code, lots of turret prefabs lying around unused in the project, and it won't even work that well since most turret swaps also require hull adaptations IRL (you cant just plop down the turret on a hardpoint and call it a day).

there are many more but I need to remember/have them pointed out to me

[pvutov]

Should add an 'inherits' field inside turrets too, to reduce duplication wrt weapons