Check that flows are focused in circuit extraction

[matulni]

As noted by @pranav97nair and @thierry-martinez in #445 , PauliExponentialDAG.from_focused_flow requires the flow to be focused but does not verify this. When the flow is obtained from OpenGraph.extract_pauli_flow, we don't need such a check since Pauli-flow extraction always returns a focused Pauli flow by construction, but this may not hold for arbitrary user-constructed flows (and following the same logic, we don't even check that the flow is well-formed). The current approach merely relies on the function name from_focused_flow to indicate that the flow should be focused (and well-formed, of course).

Another option would be to run is_well_formed and is_focused systematically, but that would lead to unnecessary checks in the common cases where flows come from extract_pauli_flow. This could be circumvented by adding a flag check_valid_flow to be set to False when performance is needed, but adding boolean flags to determine the function's behaviour is in general not a good practice.

@thierry-martinez suggests introducing FocusedPauliFlow and FocusedGFlow classes to indicate that a flow is focused at the type level (although this may render the API overly complex).

A related research question is open: are flows obtained from patterns via Pattern.extract_gflow focused ?

[matulni]

@thierry-martinez : Suggests introducing a new subclass FocusedPauliFlow that inherits from PauliFlow so that we can indicate at the type level that a flow is focused.
The difficulty is to have two different constructors for the same object, one that runs an is_focused check (e.g., when the user instantiates the object), and one which doesn't (e.g., when the flow is returned by the O(N3) algorithm). This difficulty also arises with the is well-formed property.