Transition streamline file I/O operations to latest NiBabel/DIPY

[jhlegarreta]

Following #8, it would be wise to transition all streamline file I/O operation calls to the latest NiBabel/DIPY's StatefulTractogram APIs.

It turns out that the current repository uses 3 different legacy releases or conventions from NiBabel API (e.g. calls like nb.streamlines.detect_format, tracts_file.load, nb.trackvis.read, nb.streamlines.tractogram.Tractogram, nb.streamlines.tck.TckFile, etc.). So using the latest NiBabel/DIPY releases would hide much of the complexity of using different file formats, and avoid unexpected results.

Note that the tool makes all its internal processing in voxel space, to it would be enough to send files to voxel space when loading and use in world coorss (RASMM) space when writing.

Prior to #8, tracks were being twice as large as expected and with twice the shift, and no conversion (read/write) with an anatomical reference file (.nii.gz) was being able to fix that.