CLI: add possibility to download pseudo archives and metadata files without installing

[sphuber]

This would be useful in the case where the automated command fails and a user needs to perform a manual install through aiida-pseudo install family. Here it would be good to make downloading the files as easy as possible by either providing the URLs that need to be downloaded, or by downloading them through a command (but skipping the installation part).

[mbercx]

Maybe add a --skip-install / --download-only option to the install commands?

[zooks97]

In the same line, it could be convenient to have a --data-directory flag which specifies where to download the files to (i.e. not a temporary directory). So, if a user needs to reinstall the same family at a later time for e.g. a different profile, they don't have to re-download the files each time. This would also make the MD5 checksum suggestion in #62 an even better idea as files may be corrupted or changed on disk after download and should be checked for subsequent use.

[sphuber]

The current PR adds it to the current working directory. We can add another flag in the future to add a more specific directory