Datalad DP cannot provide size for its datasets

[prioux]

When registering a collection using the DataladDataProvider, the size is always unset.

That's because the size-setting method crawls through the dataset and builds a sum of the size of all files (skipping directories and symlinks), but because the content of all the files is replaced by symbolic links, not files are found.

It seems Datalad might have a better size-fetching method, it would need to be included as part of the DataProvider API (provider_file_attributes(userfile) maybe?).