Design pspec sslbl bug in modelpacks

[sserita]

Describe the bug
The processor_spec in GST designs output by modelpacks do not properly set qubit_labels.

Pretty sure the problem is just self._sslbls here:

pyGSTi/pygsti/modelpacks/_modelpack.py

Lines 117 to 132 in d27e4e6

	def processor_spec(self, qubit_labels=None):
	"""
	Create a processor specification for this model pack with the given qubit labels.
	Parameters
	----------
	qubit_labels : tuple, optional
	A tuple of qubit labels, e.g. ('Q0', 'Q1') or (0, 1). The default
	are the integers starting at 0.
	Returns
	-------
	QubitProcessorSpec
	"""
	static_target_model = self.target_model('static', qubit_labels=qubit_labels) # assumed to be an ExplicitOpModel
	return static_target_model.create_processor_spec(self._sslbls)

But I'm in the middle of something else so noting it here and will come back to it.