unified-core module and some fixes / minor changes

[NichtStudioCode]

This pull requests adds the unified-core module. Previously, the project was lacking a way of listening to custom block placing / breaking, so it wasn't possible to register newly placed energy storages. I've decided not to put those methods into the unified-energy module, so that the project can be expanded in the future without needing to listen to both placing of energy storages and (for example) item storages.
The current idea is to implement the UnifiedBlockListener in your project and then call UnifiedEnergy.getInstance().getEnergyStoragesAt (and in the future also something like UnifiedItems.getInstance().getItemStorageAt). I've also already started to implement this in Nova. You can look at our implementation here.

I've also updated the EnergyNetworkManager#getEnergyStoragesIn method to return a map of Location, EnergyStorage and fixed a ConcurrentModificationException in the corresponding UnifiedEnergy methods