NF: added mean, axial and radial diffusivities for DTI (essential for DTI users and clinicians)
DOC: added documention
Making tests
Added tests for trace, mean, axial and radial diffusivities
ENH: Add the tensor metrics to the TensorFit object as autoattr's.
TST: Test that dti metrics get calculated on the multi-voxel case.

Also: fixed some typos from copy-pasting.
RF: all_zeros was not being used in any of these.