Make 'id' a mandatory parameter in FusSceneConverter.ConvertFrom()

[ASPePeX]

It is optional in 0.9 to be backwards compatible.
It should be forced to prevent #202.

[ASPePeX]

Since it was unclear in the last meeting, this is the line this issue is talking about.

Fusee/src/Engine/Core/FusSceneConverter.cs

Line 25 in 2a624ea

public static SceneContainer ConvertFrom(FusFile fus, string id = null)