Add --retry=10 option to handle network failures for prepare_regions.sh

[youngchanpark]

During download of some data files in a prepare_regions.sh run, the download may fail not due to the bash code, but because of a temporary network failure.
For example:

./prepare-regions -o /my/output/directory

Output

Downloading cell type : hepatocyte
Downloading cell type : iPS_15b
Downloading cell type : iPS_20b
Downloading cell type : iPS_DF_19_11
Downloading cell type : iPS_DF_6_9
Downloading cell type : keratinocyte
Downloading cell type : kidney
Downloading cell type : large_intestine
[Error] At this step something failed. The file was not created! /my/output/directory/prepare_regions_tempfiles/2021.05.06/EnsemblRegulation/large_intestine.gff.gz
[Error] Exiting.

Rerunning with the -r (reuse) option successfully completed till the end.

./prepare-regions -r -o /my/output/directory

@agilly suggested that we should add a retry option which attempts to retry downloading the failed file, so that we don't need to rerun with the -r option.