Release Name: 0.6

Notes:
version 0.6
* Added support for more flexible gradient table format (DTI only). b0 (0, 0, 0)
vector as well as b values (as 4th component) can be included in gradient table.
Thus, it allows reconstructing data with multiple high b values.
* Added option to track by 2nd or 3rd eigenvector (DTI only).
* Added track_merge command-line tool.