ENH: Remove unused parameter from method prototype

Remove unused parameter from method prototype: PET images do not have a
`b0` image. Use the `*_` to denote unused arguments for the `PET` class
and use a `kwargs` dictionary for the `DWI` case.

src/eddymotion/data/dmri.py

@@ -195,8 +195,9 @@ def to_filename(self, filename, compression=None, compression_opts=None):
                         compression_opts=compression_opts,
                     )
 
-    def to_nifti(self, filename, insert_b0=False):
+    def to_nifti(self, filename, **kwargs):
         """Write a NIfTI 1.0 file to disk."""
+        insert_b0 = kwargs.get("insert_b0", False)
         data = (
             self.dataobj
             if not insert_b0

src/eddymotion/data/pet.py

@@ -121,7 +121,7 @@ def to_filename(self, filename, compression=None, compression_opts=None):
                         compression_opts=compression_opts,
                     )
 
-    def to_nifti(self, filename, insert_b0=False):
+    def to_nifti(self, filename, *_):
         """Write a NIfTI 1.0 file to disk."""
         nii = nb.Nifti1Image(self.dataobj, self.affine, None)
         nii.header.set_xyzt_units("mm")