code: def install(packagename, save, save_dev, save_test, filename):
      
      print('Installing ', packagename)
      print(sh_pip.install(packagename))
      if not filename:
          filename = get_filename(save, save_dev, save_test)
      try:
          add_requirements(packagename, filename)
      except AssertionError:
          print('Package already pinned in ', filename)
docs: Install the package via pip, pin the package only to requirements file.
      Use option to decide which file the package will be pinned to.