Version 0.11.0

Universal representation of decay chains:

• More documentation and testing.
• Allow default class method DecayMode.from_pdgids() mimicking the default constructor DecayMode().
• Fix in logic of DecayChain.flatten().
• DecayChainViewer class adapted to make use of graphviz.
  Dependencies:
• Got rid of dependency on pydot, as the package is no longer maintained.
• Replaced with dependency on graphviz, made a requirement.
  Tests:
• Check for some expected warnings, to get rid of obvious warnings.
  Miscellaneous:
• Follow Scikit-HEP org guidelines for code development and packaging.