Correctly handle TRK format (as in trackvis)

[mdesco]

Trk files should be handled to work correctly with trackvis.

This means:

• correctly supporting the "trackvis space" (see Tractconverter + Nibabel).
• keep scalars and properties.

Originally, @mdesco wrote: If I give input to FiberCompression a *.trk saved from Dipy as in the Gallery. The program executes in 0min and does not do anything.

https://www.dropbox.com/s/bbohfcerdo5mp76/dipy_stem_L_streamlines10K.trk

e.g. fibercompression ~/Dropbox/dipy_stem_L_streamlines10K.trk bar.trk -e 0.1 -c -f

Probably due to a count of 0, which is valid in trk.