New option -fslgrad to supply DW encoding in FSL bvecs/bvals format

As discussed in issue #51.
Updates following creation of -fslgrad option

- Throw an exception if both -grad and -fslgrad are provided
- Updated exception and doxygen texts to reflect new behaviour
- Allow get_DW_scheme() to return an empty matrix if no encoding is found, but detect such an empty matrix in check_DW_scheme()
dwiextract: Ensure valid diffusion encoding is found
mrconvert: Compatibility with -fslgrad option
Allow DW gradient encodings that have fewer than 7 rows

As discussed in #61.
Merge pull request #61 from jdtournier/fsl_bvec_handling

New option -fslgrad to supply DW encoding in FSL bvecs/bvals format