Option to spread iceberg runoff (rofi) field (#13)

This change allows for spreading of the iceberg melt field according to
a climatological pattern.

A new configuration parameter is added, `rof2ocn_ice_spread` - which is
the filename for a netcdf file with a 12-monthly climatological
spreading pattern. If this parameter is not set - there is no change to
existing behaviour.

When a file is provided, the incoming `Forr_rofi` field is spread
according to the pattern supplied in the climatology of spreading.


See example OM3 100km configuration: ACCESS-NRI/access-om3-configs@3169a38

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Co-authored-by: Dougie Squire <dougie.squire@anu.edu.au>

Author: anton-seaice