refactor: introduce model and exchange factories

[wpbonelli]

• factor Model/ExchangeFactory modules out of SimulationCreate module
• add noop virtual PRT and SWF models and exchanges — allows cleaner templates, and virtual models/exchanges will be added anyway when SWF and PRT support parallel
• prep for templating for build-time model selection (see comments TODO BEGIN/END TEMPLATING)
• rename exchange create routines for templating
• move typename and filename up to BaseExchangeType
• deduplicate model registration logic, this...

    model => null() ! can be null for remote models
    if (model_ranks(n) == proc_id) then
      im = im + 1
      write (iout, '(4x,2a,i0,a)') 'GWF6', ' model ', &
        n, ' will be created'
      call gwf_cr(fname, n, model_names(n))
      model => GetNumericalModelFromList(basemodellist, im)
      model_loc_idx(n) = im
    end if
    call add_virtual_gwf_model(n, model_names(n), model)
  

  ...was previously repeated for each model type, factor out a shared routine accepting proc pointers
• consolidate add/create routines in virtual model types
• carved out of #1532
• step toward #1491
• after this, we can introduce fypp, add templates for Model/Exchange/ConnectionFactory, and update meson files to generate source from templates