Segment in SPM interface throws error.
In Segment, include bias-corrected output only if corresponding save_bias_corrected boolean is True.
This prevents to throw an error when save_bias_corrected is set to False but Segment still looks for a created bias-corrected output.
Add deprecation for old output variable modulated_input_image in Segment (to be replaced with bias_corrected_image) and removed exists=True for outputs that are predicated on input settings
Merge pull request #580 from mick-d/master

Correct Class Segment in spm.preprocess