ENH: Add 'sse' output to FSL DTIFit interface

[divetea]

Summary

Add Sum-Squared-Error to DTIFit output.

Fixes #2743 .

List of changes proposed in this PR (pull-request)

• add 'sse' to DTIFitOutputSpec
• add logic to ignore this output if the corresponding input-flag was not set

I was not able to run the tests since i cannot install all requirements on my work system.
My changes are very similar to the already existing 'save_tensor' input with 'tensor' output.
The logic to only add the output if the corresponding input-flag was defined and set to True, was only tested manually with an example dataset, since it depends on the actual output/file creation!
I did not find any tests for the 'save tensor' -> 'tensor' input-output pair, so i had no orientation how to approach this for my new output.
In fact i did not find any tests that go further than the correct assembly of the cmdline.

I think the approach to define a dictionary which maps an optional output to the corresponding input-flag is quite generic and might be useful in similar use-cases. In the old code 'save_tensor' was handled as a special case with some duplication.

Acknowledgment

• (Mandatory) I acknowledge that this contribution will be available under the Apache 2 license.

[effigies]

LGTM. Thanks.