tckmap: Tweak DEC image output

Use better intensity scaling for DEC images.
fod2metric: Alternative DEC outputs

Alternative DEC outputs: one with a unit vector per voxel, another weighted by AFD.
Also fixed the DEC combination to provide a proper AFD weighting rather than a sum of DECs.
tckmap: Fix bug in previous commit

Fixing TO output with min/max voxel-wise statistic. Not that I think these will ever be sensibly used...
fod2metric: Change to fod2fixel and fixel2voxel

Generating scalar images based on FODs is now broken into two steps:
- The FOD segmentation is handled in fod2fixel (renamed from fod2metric to better reflect the operation being performed), which now only writes fixel output images.
- fixel2voxel now has a much wider range of operations it can apply for collapsing fixel data down to a scalar image.
Also added an easy option for disabling all segmentation thresholds, so that all identified FOD lobes are retained.
Closes #129.
mrmath: Fix bug in magmax operation

The result of this operation is the value for which the magnitude is a maximum; not to be confused with absmax, which provides the maximum absolute value.
mrmath: added description of operations to help page
Merge branch 'master' into C++11

Conflicts:
	cmd/fixel2voxel.cpp
	cmd/fod2metric.cpp
	cmd/mrmath.cpp
	src/dwi/tractography/mapping/writer.h