ice_forcing: do allow 'use_leap_years' with 'atm_data_type=default'

In e81f710 (Added JRA55 forcing for gx3 grid; modified ice-forcing.F90
to now han… (CICE-Consortium#408), 2020-02-28), ice_forcing::init_forcing_atmo was
modified to allow 'atm_data_type=JRA55_gx3' to be used with
'use_leap_years', but by mistake it removed 'default' from the allowed
values of 'atm_data_type' when 'use_leap_years' is used.

Put it back so that atm_data_type=default and use_leap_years can be used
together again.

cicecore/cicedyn/general/ice_forcing.F90

@@ -283,6 +283,7 @@ subroutine init_forcing_atmo
       endif
 
       if (use_leap_years .and. (index(trim(atm_data_type),'JRA55') == 0 .and. &
+                                trim(atm_data_type) /= 'default'     .and. &
                                 trim(atm_data_type) /= 'hycom'       .and. &
                                 trim(atm_data_type) /= 'box2001'))   then
          write(nu_diag,*) 'use_leap_years option is currently only supported for'