Improve gammapy download

[Bultako]

This PR extends functionalities of gammapy download - initially added in #1369 as a CLI to address issues of PIG 4 - Setup for tutorial notebooks and data in #1419.

This PR provides gammapy download with new options:

• specify gammapy version release or commit hash in Github repo
• downloading a specific file or all content from a specific folder
• recursive/no-recursive flag to avoid content download recursively

$ gammapy download 
Usage: gammapy download [OPTIONS] COMMAND [ARGS]...

  Download datasets and notebooks.

  Download from the 'gammapy-extra' Github repository the content of
  'datasets' or 'notebooks' folders. The files are copied into a folder
  created at the current working directory.

  Examples
  --------

  $ gammapy download notebooks
  $ gammapy download datasets
  $ gammapy download --file=first_steps.ipynb notebooks
  $ gammapy download --dest=localfolder --fold=catalogs/fermi --no-recursive datasets
  $ gammapy download --hash=master notebooks

Options:
  --dest TEXT                   Folder where the files will be copied.
  --file TEXT                   Specific file to download.
  --fold TEXT                   Specific folder to download.
  --release TEXT                Release or commit hash in Github repo.
  --recursive / --no-recursive  Deactivate recursive scan of a folder.
  -h, --help                    Show this message and exit.

Commands:
  datasets   Download datasets
  notebooks  Download notebooks

[cdeil]

@Bultako - Thanks!

Do you think it would be be possible / advantageous to add tests that execute this code for gammapy download?

Generally we're trying to reduce / remove tests that hit the network because of annoying timeout failures and generally they are slow, but having non-trivial code without automated tests can also be problematic.

Tests can be added later for this, merge whenever you like.

[Bultako]

@cdeil
Yes, it would be good to have some tests. At least for small files or datasets that we think there will always be available. Now that we can download specific files, it's possible.
👍