BIDSDataGrabber doesn't populate outputspec when outfields is defined

[mvdoc]

Summary

When defining custom fields BIDSDataGrabber doesn't add the corresponding traits to output_spec, so it's not possible to link nodes in a workflow.

Actual behavior

In [1]: from nipype.interfaces.bids_utils import BIDSDataGrabber

In [2]: bg = BIDSDataGrabber(infields=['infield1', 'infield2'], outfields=['outfield1', 'outfield2'])

In [3]: bg._outputs()
Out[3]: 

Expected behavior

In [1]: from nipype.interfaces.bids_utils import BIDSDataGrabber

In [2]: bg = BIDSDataGrabber(infields=['infield1', 'infield2'], outfields=['outfield1', 'outfield2'])

In [3]: bg._outputs()
Out[3]: 

outfield1 = <undefined>
outfield2 = <undefined>

I started working on a fix for this, and it seems to me that perhaps subclassing IOBase instead of BaseInterface might be a solution. But I was wondering what you think it would be better.