Show_tree output is not printed for old ext files

[priscavdsluis]

Describe the bug
For the new ext file, the files reference in here are printed out with the show_tree function.

To Reproduce (example)

# initialize mdu file and update settings
mdu_file = os.path.join(dir_output, f'{model_name}.mdu')
mdu = hcdfm.FMModel()

# add the external forcing files (.ext)
mdu.external_forcing.extforcefile = ext_file_old
mdu.external_forcing.extforcefilenew = ext_new

# visualize the model tree
mdu.show_tree()

output:

  c:\DATA\dfm_tools\docs\notebooks\Vietnam_model\Vietnam.mdu
    Geometry
     ∟ c:\DATA\dfm_tools\docs\notebooks\Vietnam_model\Vietnam_net.nc
    ExternalForcing
     ∟ c:\DATA\dfm_tools\docs\notebooks\Vietnam_model\Vietnam_old.ext
     ∟ c:\DATA\dfm_tools\docs\notebooks\Vietnam_model\Vietnam_new.ext
       ∟ Boundary
         ∟ c:\DATA\dfm_tools\docs\notebooks\Vietnam_model\Vietnam.pli
         ∟ c:\DATA\dfm_tools\docs\notebooks\Vietnam_model\tide_Vietnam_tpxo80_opendap.bc
       ∟ Boundary
         ∟ c:\DATA\dfm_tools\docs\notebooks\Vietnam_model\Vietnam.pli
         ∟ c:\DATA\dfm_tools\docs\notebooks\Vietnam_model\waterlevelbnd_Vietnam_CMEMS.bc
       ∟ Boundary
         ∟ c:\DATA\dfm_tools\docs\notebooks\Vietnam_model\Vietnam.pli
         ∟ c:\DATA\dfm_tools\docs\notebooks\Vietnam_model\salinitybnd_Vietnam_CMEMS.bc
       ∟ Boundary
         ∟ c:\DATA\dfm_tools\docs\notebooks\Vietnam_model\Vietnam.pli
         ∟ c:\DATA\dfm_tools\docs\notebooks\Vietnam_model\temperaturebnd_Vietnam_CMEMS.bc
       ∟ Boundary
         ∟ c:\DATA\dfm_tools\docs\notebooks\Vietnam_model\Vietnam.pli
         ∟ c:\DATA\dfm_tools\docs\notebooks\Vietnam_model\uxuyadvectionvelocitybnd_Vietnam_CMEMS.bc
    Output
     ∟ c:\DATA\dfm_tools\docs\notebooks\Vietnam_model\Vietnam_obs.xyn

Expected behavior
The file paths referenced in the old external forcing file are also listed in the output.

Version info (please complete the following information):

• Version 0.5.2

Additional context
Probably the is_intermediate_link needs to set to true for forcing objects in the old ext file.