Gemini/GNIRS: Replace ``self.dispname`` with another mechanism for specifying echelle order information

[tbowers7]

The GNIRS spectrograph has multiple echelle gratings, and therefore the echelle-specific properties in the Class need to return different value based on which grating was used:

PypeIt/pypeit/spectrographs/gemini_gnirs.py

Lines 533 to 594 in 66b3791

	@property
	def norders(self):
	"""
	Number of orders for this spectograph.
	"""
	self.check_disperser()
	if '10/mmLBSX' in self.dispname:
	return 4
	elif '32/mm' in self.dispname:
	return 6
	else:
	msgs.error('Unrecognized disperser')
	@property
	def order_spat_pos(self):
	"""
	Return the expected spatial position of each echelle order.
	"""
	self.check_disperser()
	if '10/mmLBSX' in self.dispname:
	return np.array([0.050, 0.215, 0.442, 0.759])
	elif '32/mm' in self.dispname:
	#ToDo: create self.date similar to self.dispname and use that to decide which numbers to use
	## Old data, i.e. before 2011
	#return np.array([0.241211 , 0.3173828, 0.387695, 0.456054, 0.530273, 0.640625])
	##New data
	return np.array([0.2955097 , 0.37635756, 0.44952223, 0.51935601, 0.59489503, 0.70210309])
	else:
	msgs.error('Unrecognized disperser')
	@property
	def orders(self):
	"""
	Return the order number for each echelle order.
	"""
	self.check_disperser()
	if '10/mmLBSX' in self.dispname:
	return np.arange(6,2,-1, dtype=int)
	elif '32/mm' in self.dispname:
	return np.arange(8,2,-1,dtype=int)
	else:
	msgs.error('Unrecognized disperser')
	@property
	def spec_min_max(self):
	"""
	Return the minimum and maximum spectral pixel expected for the
	spectral range of each order.
	"""
	# TODO: Why aren't these numbers in fraction of the detector
	# size instead of the pixel number?
	self.check_disperser()
	if '10/mmLBSX' in self.dispname:
	spec_max = np.asarray([1022, 1022, 1022, 1022])
	spec_min = np.asarray([450, 0, 0, 0])
	return np.vstack((spec_min, spec_max))
	elif '32/mm' in self.dispname:
	spec_max = np.asarray([1022, 1022, 1022, 1022, 1022, 1022])
	spec_min = np.asarray([512, 280, 0, 0, 0, 0])
	return np.vstack((spec_min, spec_max))
	else:
	msgs.error('Unrecognized disperser')

The problem with the present implementation is the use of self.dispname -- which is defined in config_specific_par() -- and how it gets propagated throughout the remainder of the PypeIt code base. For instance, the solution to #1977 (in PR #2013) involved additional gymnastics to satisfy Gemini/GNIRS that ends up calling config_specific_par().

This is the only spectrograph to use self.dispname, and it would be helpful to find another mechanism for specifying the echelle order information. In fact, there is a TODO in the GNIRS class about finding a way around this hack:

PypeIt/pypeit/spectrographs/gemini_gnirs.py

Lines 470 to 472 in 66b3791

	# TODO This is a hack for now until we figure out how to set dispname
	# and other meta information in the spectrograph class itself
	self.dispname = self.get_meta_value(scifile, 'dispname')