Add `esmvaltool develop` command group

[alistairsellar]

Is your feature request related to a problem? Please describe. Plan agreed with @bouweandela:

• Create esmvaltool develop command, by adding new class to esmvaltool_commands in setup.py. Initial subcommands are:
  • esmvaltool develop compare, utilising utils/testing/regression/compare.py
  • esmvaltool develop test_recipe, utilising utils.testing.recipe_settings.install_expand_run.py,
  • esmvaltool develop recipe_filler, utilising 'utils.recipe_filler.py`
• Move the above python files to new folder esmvaltool/utils/develop to have them organised by command group

Note that this will also fix #3437

Would you be able to help out? On it now