downloader: resume interrupted downloads about cernopendata-client HOT 3 OPEN

@tiborsimko

if the file is partial, it would check whether the user used --resume option, and if yes, then would continue from that point, and if no, it would ask the user whether the resume or redownload is wanted.

I have a couple of musings.

1. We will know if a file is partially downloaded when it is not matching the checksum and size from the remote file.

2. How we plan to handle the resuming of download?
   We will go like requests -> pycurl -> xrootd and use inbuilt functionality from the respective library or you have some different approach in mind?

from cernopendata-client.

@katilp comment link on root forum: https://root-forum.cern.ch/t/running-time-dependence-on-cluster-distance-from-cern-for-jobs-on-gke-cluster-open-data/42447

from cernopendata-client.

Resuming of downloads

1. Requests - #109
2. Pycurl - #117
3. Xrootd -

from cernopendata-client.