FNIRT output specification issues

[djarecka]

@htwangtw - I was debugging failing tests and it looks like there are several field in the FNIRT output spec (e.g. field_file) that have in_file as the only requirements, but this is not right.
In nipype one can specify full name by providing a string or set it to True in order to use the default nipype template. @effigies suggested that we don't have to support both, we could just allow for some user defined template.

Let me know if you have thoughts about it. Don't remember if we discussed similar cases

[htwangtw]

I agree with @effigies that we should just pick one.
I am inclined to ask for user defined template, as that would be more consistent with the docstring of FSL FNIRT. Tried to play around with it today but no success with fixing it.

[effigies]

@ghisvail Is this resolved?

[ghisvail]

There is no output spec for FNIRT anymore as a result of #41.

Everything is now derived from in_file as template for the default name in the input spec. This was necessary to get the tests green again.