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
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.
Merge branch 'master' into C++11

Conflicts:
	cmd/dwi2response.cpp
	lib/math/sinc.h
	src/dwi/shells.cpp