Merge pull request #21 from MrBago/peak_finding

RF - Make peak finding return zero peaks when there are no good peaks to...
Merge pull request #274 from Garyfallidis/peak_directions_update

Peak directions update
Merge pull request #285 from MrBago/segfault_issue

Bad seeds cause segfault in EuDX
NF - added affine argument to streamline tools.
TST - added test for affine with streamline_mapping
RF - make coordinate mapping/inversion more clear
BF - fix target function
RF - cleaned up seeds_from_mask and added affine argument
NF - added include argument to target function
DOC - first draft of streamline tools tutorial
DOCS - make streamline_tools render properly as html
NF - added fetcher and reader for stanford labels data
TST - added tests for data fetcher
DOCS - improvements to streamline_tools example
RF - move streamline_mapping to vox2track
RF - import python and cython code into one utils module
RF - rework much of utils
TST - added tests for move_streamlines and affine_for_trackvis
NF - added affine keyword to eudx classes and tests
NF - add affine attribute to markov tracking
RF - rework utils to emphasis affine and deprecate voxel_size