lookup()/lookup_fd() does not filter out flag diacritics

[bowersd]

The documentation almost says that lookup() does something with flag diacritics (https://github.com/hfst/python/wiki/Symbols), but this has been left as a TODO. Currently flag diacritics appear in the output from lookup()

Meanwhile, lookup_fd() is stated to handle flag diacritics as epsilons, but HfstTransducer objects apparently do not have an attribute lookup_fd().