Added bottle volume constant and two helper methods to FluidContainerRegistry

- Added bottle volume constant (250mb). Technically the volume should be BUCKET_VOLUME/3 based on how many bottles you get out of a cauldron (holds 1 bucket of water and you can fill 3 bottles before it is empty). But considering that working with 333mb would be a major pain 250mb seems a good compromise.

- Added drainFluidContainer() and getContainerCapacity() helper methods. Currently you have to iterate over the FluidContainerData returned by getRegisteredFluidContainerData to get the capacity for a container or to get the empty container item for a full one.