Add a script to diff namelists

[ScottWales]

diffnml will create a diff of two namelists, using f90nml as its parser

$ diffnml old/input.nml new/input.nml
&ocean_velocity_nml%truncate_verbose:
    <   False
    >   True
&ocean_velocity_nml%truncate_velocity:
    <   False
    >   True