Tractseg

* bugfix: make download of weights from zenodo work again
* bugfix: change sklearn package name to scikit-learn in setup.py

* bugfix: reorient streamline in plot_tractometry_results was not in voxel space
* bugfix: use correct interpolation with flirt on binary masks
* xvfb for plotting

* Bugfix when tracking in non-MNI space
* **Interface change**: make `tck` the default tracking format since `trackvis` api is no longer supported by nibabel
* use spline interpolation instead of trilinear interpolation in flirt for option `--preprocess`
* improve weights download

* Update documentation
* fixed bug with newer fsl bet (thanks to CarolynMcNabb)
* fix bug with import of `compress_streamline()` from dipy

* Move cython tracking to `cython_tracking` branch. It is not part of the main branch and pip package anymore because building the cython binaries for all possible operating system is a lot of pain. If people want to use it they can install from the `cython_tracking` branch directly which will automatically compile the cython code on their sytem.

* Update documentation
* Bugfix when tracking in non-MNI space
* Use Cython for Tracking. This will speed up tracking by roughly 2x and only require 1 CPU core instead of all CPU cores. Thanks to StavrAspa and eliaskoromilas for this contribution.