'Return the precalculated depth of a block within the block_structure:
Arguments:
block_structure: a BlockStructure instance
block_key: the key of the block whose depth we want to know
Returns:
int'

@classmethod
def get_block_depth(cls, block_structure, block_key):

    return block_structure.get_transformer_block_field(block_key, cls, cls.BLOCK_DEPTH)
