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.
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()
small app 'test_strides' to check Image::Stride::get_nearest_match()
FixelMetric class: rename member 'amplitude' to 'size'
SIFT: Change some cmdline options and outputs

Trying to remove some old references to FOD peak amplitudes / integrals, and stick with a more generic definition of 'fibre density'
Fix compilation after previous two commits

a3f26019 and 3ef0ba5a introduced some compilation problems due to Git and myself having a disagreement.
tckmap: Fix -contrast scalar_map with 4D image

Another case of those pesky 3D images that are actually 4D with dim(3)=1.
Fix bug in previous commit
Tractography::Mapping: Fix a couple of bugs

These were found while modifying the C++11 branch a while back, but weren't corrected here.
Strides::sanitise(): minor changes to argument names & doc
Remove test_strides test app
Merge pull request #144 from jdtournier/stride_sanitise_fix

Stride sanitise fix
Merge branch 'master' into C++11

Conflicts:
	cmd/fixel2voxel.cpp
	lib/image/stride.cpp
	lib/image/stride.h
	src/dwi/tractography/SIFT/model_base.h
	src/dwi/tractography/SIFT/sift.cpp
	src/dwi/tractography/mapping/mapping.cpp
	src/dwi/tractography/mapping/voxel.h