'This is an alternative to the standard transform method.
Returns a list of filter functions to be used for filtering out
any unwanted blocks in the given block_structure.
In addition to the commonly used methods listed above, the following
methods are commonly used by implementations of transform_block_filters:
create_universal_filter
create_removal_filter
Note: Transformers that implement this alternative should be
independent of all other registered transformers as they may not
be applied in the order in which they were listed in the registry.
Arguments:
usage_info (any negotiated type) - A usage-specific object
that is passed to the block_structure and forwarded to all
requested Transformers in order to apply a
usage-specific transform. For example, an instance of
usage_info would contain a user object for which the
transform should be applied.
block_structure (BlockStructureBlockData) - A mutable
block structure, with already collected data for the
transformer, that is to be transformed in place.'

@abstractmethod
def transform_block_filters(self, usage_info, block_structure):

    raise NotImplementedError
