[h5_to_nx] fix: insure no links are created if only one created (if only one acquisition to convert)

[patch-nx] due to limitation on h5pt virtual dataset it is preferable to embed the data in the file to avoid bugs and limitation (see issue #40: #40)

This reverts commit 6ce70a7f.

/close #3277

Anyway this is safer to have a clear definition of this