add small and big delta

This is a very small pull request, I found that even if gradients.py had as parameters small_delta and big_delta it didn't save them. So even if you had set the deltas using:

gtab = gradient_table(bvals, bvecs,big_delta=0.0577, small_delta=0.0454)

when you type:

gtab.big_delta

the output was always None.
This PR fix this bug.