Add logn back to the cable nml (#493)

# CABLE

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## Description

Add the ```logn``` variable back into the namelist. This was an
unintentional consequence of merged PR #460- existing configurations
that have the now superfluous ```logn``` in the cable namelist will
fail. The variable won't do anything, but needs to remain until
configurations are updated.

Fixes #492 

## Type of change

Please delete options that are not relevant.

- [x] Bug fix

offline/cable_driver.F90

@@ -294,6 +294,7 @@ PROGRAM cable_offline_driver
        patchout, &
        check, &
        verbose, &
+       logn, &
        leaps, &
        fixedCO2, &
        spincasainput, &