CDMS: fix `MOLWT` column starting position #3716

Job	Run time
egg_info with Python 3.9	9s
Code style checks	20s
oldest version for all dependencies	1m 28s
Python 3.10 with all optional dependencies (MacOS X)	2m 12s
Python 3.11 with mandatory dependencies (Windows)	2m 15s
	6m 24s

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