Remove dependence of Python system tests on nco module

[samsrabin]

These system tests fail without nco loaded, which complicates testing, especially on systems without it installed:

• test_allInfo
• test_modifyMesh

The issue is here:

CTSM/python/ctsm/test/test_sys_mesh_modifier.py

Lines 83 to 94 in bfc5c31

	# This could also alturnatively be done, by using the stored SCRIP grid file for the resolution under CESM inputdata
	ncks_cmd = (
	f"ncks --rgr infer --rgr scrip={self.scrip_file} {fsurdat_in} {self.metadata_file}"
	)
	try:
	subprocess.check_call(ncks_cmd, shell=True)
	except subprocess.CalledProcessError as e:
	err_msg = (
	f"{e} ERROR using ncks to generate {self.scrip_file} from "
	+ f"{fsurdat_in}; MOST LIKELY SHOULD INVOKE module load nco"
	)
	sys.exit(err_msg)

This should be rewritten to pure Python.