Allow LuaEntity.speed to read/change speed for projectiles

[calcwizard]

Please allow the speed of a projectile entity to be read and written during control, preferably through LuaEntity.speed or a similar method.
I would like to set the speed of an artillery projectile after being fired (through on_trigger_fired_artillery). Their speed can be set when created through LuaSurface.create_entity, but it requires a target entity (artillery flare), which can be expensive to find with the range of artillery. Some projectiles don't even have target entities (like grenades).

Thanks.

[Bilka]

This was added for the next version.

[calcwizard]

Thanks.