ENH - add warning in ``PN_logreg`` for unhandled case

[Badr-MOUFAD]

I think this is a dangerous design as it silences eventual errors/bugs.
I suggest raising a warning in this case.

celer/celer/PN_logreg.pyx

Lines 426 to 428 in 83c4628

	else:
	pass
	# TODO what do we do in this case?