Change N4 dimension input to use short tag

Using short tag will allow use of newest ANTs releases as well as provide backward compatibility as only the long tag changed. Also removed conflicting `requires` tags.

Author: blakedewey

nipype/interfaces/ants/segmentation.py

@@ -223,7 +223,7 @@ def _list_outputs(self):
 
 
 class N4BiasFieldCorrectionInputSpec(ANTSCommandInputSpec):
-    dimension = traits.Enum(3, 2, argstr='--image-dimension %d',
+    dimension = traits.Enum(3, 2, argstr='-d %d',
                             usedefault=True,
                             desc='image dimension (2 or 3)')
     input_image = File(argstr='--input-image %s', mandatory=True,
@@ -237,8 +237,7 @@ class N4BiasFieldCorrectionInputSpec(ANTSCommandInputSpec):
     bspline_fitting_distance = traits.Float(argstr="--bspline-fitting %s")
     bspline_order = traits.Int(requires=['bspline_fitting_distance'])
     shrink_factor = traits.Int(argstr="--shrink-factor %d")
-    n_iterations = traits.List(traits.Int(), argstr="--convergence %s",
-                               requires=['convergence_threshold'])
+    n_iterations = traits.List(traits.Int(), argstr="--convergence %s")
     convergence_threshold = traits.Float(requires=['n_iterations'])
     save_bias = traits.Bool(False, mandatory=True, usedefault=True,
                             desc=('True if the estimated bias should be saved'