Use HashMap of SkyModels instead of a single SkyModel in order to avoid a null pointer exception when more than one world is loaded at once

[Tank-Missile]

~~There's 1 issue currently that I have not solved, and that is when a portal is opened, the sky is default instead of the changed sky model. This is due to the sky model not being created until going through a portal.~~

I fixed the above issue by using EntityJoinWorldEvent in StellarAPI's StellarAPINetworkManager. Hopefully this will do.