prepopulate private attribute for image mask; rearrange abstract method

src/roiextractors/segmentationextractor.py

@@ -32,7 +32,7 @@ class SegmentationExtractor(ABC):
     """
 
     def __init__(self):
-        """Create a new SegmentationExtractor from specified data type (unique to each child SegmentationExtractor)."""
+        """Create a new SegmentationExtractor for a specific data format (unique to each child SegmentationExtractor)."""
         self._sampling_frequency = None
         self._times = None
         self._channel_names = ["OpticalChannel"]
@@ -44,6 +44,7 @@ def __init__(self):
         self._roi_response_deconvolved = None
         self._image_correlation = None
         self._image_mean = None
+        self._image_mask = None
 
     @abstractmethod
     def get_accepted_list(self) -> list:
@@ -67,6 +68,17 @@ def get_rejected_list(self) -> list:
         """
         pass
 
+    @abstractmethod
+    def get_image_size(self) -> ArrayType:
+        """Get frame size of movie (height, width).
+
+        Returns
+        -------
+        no_rois: array_like
+            2-D array: image height x image width
+        """
+        pass
+
     def get_num_frames(self) -> int:
         """Get the number of frames in the recording (duration of recording).
 
@@ -204,17 +216,6 @@ def get_background_pixel_masks(self, background_ids=None) -> np.array:
 
         return _pixel_mask_extractor(self.get_background_image_masks(background_ids=background_ids), background_ids)
 
-    @abstractmethod
-    def get_image_size(self) -> ArrayType:
-        """Get frame size of movie (height, width).
-
-        Returns
-        -------
-        no_rois: array_like
-            2-D array: image height x image width
-        """
-        pass
-
     def frame_slice(self, start_frame: Optional[int] = None, end_frame: Optional[int] = None):
         """Return a new SegmentationExtractor ranging from the start_frame to the end_frame.