[ENH] Add overwrite option to transform_images (#509)

* Add overwrite option to transform_images.

* Apply suggestions from code review

nimare/transforms.py

@@ -18,7 +18,7 @@
 LGR = logging.getLogger(__name__)
 
 
-def transform_images(images_df, target, masker, metadata_df=None, out_dir=None):
+def transform_images(images_df, target, masker, metadata_df=None, out_dir=None, overwrite=False):
     """Generate images of a given type from other image types and write out to files.
 
     Parameters
@@ -37,6 +37,8 @@ def transform_images(images_df, target, masker, metadata_df=None, out_dir=None):
     out_dir : :obj:`str` or :obj:`None`, optional
         Path to output directory. If None, use folder containing first image
         for each study in ``images_df``.
+    overwrite : :obj:`bool`, optional
+        Whether to overwrite existing files or not. Default is False.
 
     Returns
     -------
@@ -84,7 +86,11 @@ def transform_images(images_df, target, masker, metadata_df=None, out_dir=None):
         # Get converted data
         img = resolve_transforms(target, available_data, new_masker)
         if img is not None:
-            img.to_filename(new_file)
+            if overwrite or not op.isfile(new_file):
+                img.to_filename(new_file)
+            else:
+                LGR.debug("Image already exists. Not overwriting.")
+
             images_df.loc[images_df["id"] == id_, target] = new_file
         else:
             images_df.loc[images_df["id"] == id_, target] = None