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
Minor change to lib/thread.h - should sort out compilation issues on MacOSX
dwi2response tweaks

- Ensure that data for at least lmax=4 is provided, abort otherwise
- Print b-value shell mean values and volume counts when -debug flag is set
- Use BufferPreload for DWI data
Image::Stride::sanitise() fix

Fix the case where a limited constraint on the image strides has been provided, and the remaining strides are to be determined based on a reference image.
This was otherwise causing an error when using Image::Stride::contiguous_along_axis(3) if the template InfoType had been expanded from 3D to 4D (and hence stride[3]=0).
Fixes #142.
Restored sinc interpolation

Sinc interpolation had been fitted with a couple of warnings, but on testing it appears to be doing as it should.
The residual problems may have been related to its use in the context of EPI distortion correction, which is now defunct.
Also some minor cleanups in associated code.
Closes #138.
Final changes to get clean compile on MacOSX

Also fixes issue with File dialog not allowing selection of .nii.gz files.
Merge branch 'master' into C++11

Conflicts:
	cmd/dwi2response.cpp
	lib/math/sinc.h
	src/dwi/shells.cpp
dirmerge: now works (renamed from dirsplice)
Merge branch 'C++11' into ROI_tool
dwi2response: Require lmax >= 4
Fix small error in track dixel mapping

Unlikely to have caused many issues, as a single streamline would need to traverse the exact same dixel more than once.
tckmap: Small speedup to TOD mapping
Image::Stride: further fix to sanitise() and contiguous_along_axis()
Image::Stride: further fix to sanitise() and contiguous_along_axis()