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