Missing entry for 'force_retreat' namelist option in namelist_definition_cism.xml

[mpetrini-norce]

'force_retreat' is a CISM namelist option introduced (to my knowledge) in NorESM by @hgoelzer to prevent the ice sheet to expand outside the present-day observed ice mask (or, more in general, outside a mask given by the ice_fraction_retreat_mask input variable). For some reason I don't remember, this variable was included in the namelist_definition_cism.xml via SourceMods. To minimise the use of SourceMods, I suggest that this variable should be added to cime/config/namelist_definition_cism.xml (at least in NorESM2.5-alpha05 and future versions). Two important things I would like to highlight:

1. The original 'force_retreat' option was a 'logical', whereas with multiple ice sheets it has become an integer (0 = do not force retreat, 1 = force retreat for all types of ice, 2 = force retreat for floating or weakly grounded ice). So introducing this variable would mean having something like this in here

CISM-wrapper/cime_config/namelist_definition_cism.xml

Line 2820 in 489304b

<entry id="force_retreat">

    <type>integer</type>

    <category>cism_config_ho_options</category>

    <group>ho_options</group>

    <values>

      <value icesheet="gris">0</value>

      <value icesheet="ais" >0</value>

    </values>

    <desc> Flag that indicates whether or not mask SMB outside present-day grid.
      0=no force retreat, 1=force retreat where indicated by retreat_mask,
      2=force retreat for floating ice where indicated by retreat mask
      Default: 0 for Greenland, 0 for Antarctica
    </desc>

</entry>

2. If force_retreat takes the value of 1 or 2, CISM input file should contain a variable called ice_fraction_retreat_mask which is telling where ice should be removed. Currently both the standard input files for Greenland and Antarctica don't include this variable - which is fine, although perhaps we might want to have it included by default in the future? Another important thing to note: previously, ice was removed wherever the ice_fraction_retreat_mask was equal to 0 - it is the opposite now, with ice being removed for any value of ice_fraction_retreat_mask > 0.

@hgoelzer @mvertens