🐛 `obs_vars` not being caught

[chris-ashe]

Describe the bug

When running an old version on main with large_tokamak.IN.DAT. I get this:

File "/home/mn3981/PROCESS/process/input.py", line 2230, in parse_input_file
    raise ProcessValidationError(error_msg)
process.exceptions.ProcessValidationError: Unrecognised input 'i_coolant_pumping' at line 503 of input file.

Even though i_coolant_pumping is in obsolete_vars.py
The --update-obsolete flag is working.