scl_slope = 0 should not scale

[kleinschmidt]

The NIfTI spec says that the slope/intercept should be used for nonzero slope:

/ DATA SCALING:
   ------------
   If the scl_slope field is nonzero, then each voxel value in the dataset
   should be scaled as
      y = scl_slope  x + scl_inter
   where x = voxel value stored
         y = "true" voxel value

This could be checked when creating the header struct, setting the slope to 1f0 if its zero, or adding a type parameter to the nifti struct and dispatching on that for getindex.

[kleinschmidt]

(or I could keep just using the .raw field like I have been doing...)