Reorder initialization in tck2connectome.cpp

Also fix reference to depreciated command name mrprep4connectome (now labelconfig)
Add map_streamlines() function to SIFT::ModelBase class
Add explicit copy constructor to Tractography::Streamline class

Previously this may have matched to the std::vector< Point<float> > parameter constructor rather than automatically generating a copy constructor.
New convenience functions for streamline mapping

Functions determine_upsample_ratio() determine how densely to up-sample streamlines data before mapping to voxels, based on the target voxel grid, the streamlines step size, and how precise the mapping should be (e.g. if looking to quantify length through the voxel, the streamlines should be sampled more densely than if just logging streamline counts)
Fix compilation after previous commit bc6ddf35
New typedef Tractography::Connectomics::NodePair

Cleans up some of the connectome code a little bit.
Also used Tractography::Streamline<> class throughout.
Playing with mutables and consts in tractography mapping code

Needed to allow these classes to operate on streamlines data in a const way, so class members that really only store temporary data have been made mutable.
Also changed TrackMapperBase::info to a const member instead of a const reference, to allow constructing image to go out of scope.
afdconnectivity: New capabilities

Previously, afdconnectivity would select fixels based on mapping a track file corresponding to a bundle of interest. This may perform poorly in regions of partial volume, or where erroneous streamlines take a different path between the target regions of interest.
The command now has the capability to first read a whole-brain fibre-tracking data set; by doing so, for each fixel traversed by the bundle of interest, only some fraction of the AFD in that fixel is contributed to the result; this fraction is determined by the streamlines density provided by the bundle of interest, divided by the streamlines density in that fixel in the whole-brain tractogram.
Therefore if only a small part of the fixel corresponds to the bundle of interest, only a small part of its AFD contributes.