ENH: adding TRK <=> TCK streamlines conversion scripts

[MarcCote]

This PR adds two small scripts to convert streamlines from one format (TRK or TCK) to the other.

We might want to wait for #601.

[codecov-io]

Codecov Report

Merging #606 into master will decrease coverage by 0.4%.
The diff coverage is 19.35%.

@@            Coverage Diff            @@
##           master    #606      +/-   ##
=========================================
- Coverage    90.3%   89.9%   -0.41%     
=========================================
  Files          87      89       +2     
  Lines       10813   10875      +62     
  Branches     1793    1799       +6     
=========================================
+ Hits         9765    9777      +12     
- Misses        719     769      +50     
  Partials      329     329

Impacted Files	Coverage Δ
nibabel/cmdline/tck2trk.py	18.91% <18.91%> (ø)
nibabel/cmdline/trk2tck.py	20% <20%> (ø)

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[coveralls]

Coverage decreased (-0.4%) to 92.929% when pulling de68635 on MarcCote:enh_conversion_scripts into 586c0e0 on nipy:master.

[yarikoptic]

FWIW #601 is merged now ;-)

[MarcCote]

Not sure why the coverage has decreased.

[effigies]

@MarcCote Is this ready to go?

[MarcCote]

Yes, it is. There is just the coverage that slightly decreases but I don't know why. Let me know if you want me to rebase.

[effigies]

LGTM. Will merge this afternoon (EDT) in case anybody wants time to review.