Goal of the PR

Per this comment from @inmzhang and the discussion/poll that followed it seems like the active developers in this repository prefer a functional-style rather than an inheritance based style. This means that instead of

class Plaquette:
    pass

class EmptyPlaquette(Plaquette):
    pass

class ZInitialisationPlaquette(Plaquette):
    pass

# many more

with a Plaquette class whose sole purpose is to provide a common interface between plaquette, everyone voted for a functional-style:

class Plaquette:
    pass

def empty_plaquette(qubits: PlaquetteQubits) -> Plaquette:
    pass

def z_initialisation_plaquette(qubits: PlaquetteQubits) -> Plaquette:
    pass

# many more

with Plaquette being the only class used, and sub-classes are replaced by functions that initialise Plaquette instances.

How was it done?

This change is mostly (entirely?) cosmetic, as the sub-classes of Plaquette were only initialising correctly their Plaquette subclasses. This shows quite well in this PR: the changes are quite small and can be summarised in 2 points:

• Change the subclass name by the function name. The new functions have exactly the same parameters as their corresponding Plaquette subclass, so this change can be done using a "dumb" tool (i.e., just search & replace, no need for code-aware tools).
• Change the subclasses to functions:

  class [CLASSNAME](Plaquette):
      def __init__(self, [PARAMETERS]):
          [CODE]
          super().__init__([SUPER_PARAMETERS])

  becomes

  def [SNAKE_CASE(CLASSNAME)](PARAMETERS) -> Plaquette:
      CODE
      return Plaquette([SUPER_PARAMETERS])

  with a slight variation for classes that were 2 or more levels under Plaquette in the inheritance hierarchy (e.g. ZRoundedInitialisationPlaquette -> ZInitialisationPlaquette -> Plaquette) for which the super().__init__ call should be replaced with their superclass corresponding function.