Add file download validation

When a file is being downloaded, two metadata files are created for size and hash (as possible).

I applied this validation framework to the files I know (structure files, SCOP installations, Ecod installation). In case anybody knows something else where downloaded files can be validated, please advise.

I did not implement the hash code at least yet, because I don't know a place where a hash file is distributed with the resource file. In case anybody knows some example, please advise.

@sbliven was proposing to download to a temporary file before moving the load to the destination file. I found that FileDownloadUtils.downloadFile() does. Therefore, no need to do it ourselves. again, in case anybody knows a place which downloads files but does not use FileDownloadUtils.downloadFile() for download, please advise.

fixes #980.