Add BlockStorage::getBlockDataId to obtain BlockDataID from identifier

This might be useful for accessing block data from Python. @holzer I guess the previous Python backend just kept a dict of the BlockDataIDs it allocated.

Doing this with an unordered_map would be more efficient but this is probably not important here. Also this version returns the first BlockDataId with this identifier which might not be unique (but should be when used in a normal way).