Sift filtering output is not saved

[alexpron]

When testing the whole pipeline the sift filtering node executes correctly but output tractogram is not visible

[alexpron]

@all-contributors please add @davidmeunier79 for code review, tests, nipype expertise, neuroimaging expertise

[allcontributors]

@alexpron

I've put up a pull request to add @davidmeunier79! 🎉

[alexpron]

@all-contributors please add @melinacordeau for tests and associated MRI data and advice regarding workflow design

[allcontributors]

@alexpron

I've put up a pull request to add @melinacordeau! 🎉