MinimumChordalCompletion crashes

[NicoRenaud]

MinimumChordalCompletion crashes if the graph does not contain chordless cycles.
Then stack fails here

qreorder/qreorder/classical/min_chordal_completion.py

Line 155 in f5cb4c4

A = np.stack(all_coefficients)

We could add something like

if len(all_coefficients) == 0:
     return <something>
     raise Warning( .... )

to make the error a bit more understandable