temporary directories on OSX

[djarecka]

Summary

There are several issues with the way OSX handles temporary dictionary, i.e. /var -> private/var, see discussion in #2380.
The problem with test_nilearn was fixed, but this might come back with various interfaces (report.rst might also not contain all expected info)

[mgxd]

related: nipy/heudiconv#231

[djarecka]

#2639 might solve the problem permanently, but let's observe a bit longer.

If we notice any new problems, we probably should try using realpath in File, Directory and ImageFile.

[satra]

@djarecka - cannot use realpaths in File as they would resolve symlinks to their original location, thereby removing any effects of copyfile=False in traits.

i was ok with output directory of a node using realpath in this PR, but this is something to keep an eye out for.

[djarecka]

@satra ok, thanks for the comment, didn't think about copyfile=False. let's see if the problem come back

[djarecka]

#2676 reporting problem on Gentoo might be related (I thought that this problems are only with OSX)