use empty sets to represent the empty AP subset

value None should be reserved for special use and also is not compatible with the set container of atomic propositions (for labels that are non-empty sets).

tuple2fts converts None labels to empty sets, because they are more convenient for manual typing (and clear to read). Note that it does not deviate from the previous comment, because None is used as a shorthand while specifying inputs to tuple2fts, which is the usual Python convention to use the default value (here it is set()).
Simple is better than complex.
attribute _state_label_def has moved

from class LabeledStates (which doesn't exist any more) to class LabeledDiGraph
always form node label, until graph2dot improved

attribute _form_node_label of class LabeledStates no longer exists (class has been removed)
use braces instead of brackets for utterable labels

to typeset AP labels similarly to the common mathematical notation for sets
Merge branch 'switched' of https://github.com/tulip-control/tulip-control into switched