Bring hard-links of gzipped files back

[oesteban]

In #295 we discontinued hard-linking gzipped files to address poldracklab/fmriprep#1480. The request here would be to bring those back whenever possible (see https://github.com/poldracklab/niworkflows/pull/295#issuecomment-459441516).

[effigies]

I found a relevant StackOverflow thread for Python code to detect filename/mtime metadata, which could be useful in deciding whether to skip the extremely cautious unzip-rezip approach we're now taking.