Fix variable typo, initialize sample_frames to [] instead of None

The later fixes a bug as the rest of the class assumes sample_frames
is a list.

The former is currently dead code.

ml_tools/datasetstructures.py

@@ -81,7 +81,7 @@ def __init__(
         self.median_mass = np.median(frame_mass)
         self.mean_mass = np.mean(frame_mass)
         self.ffc_frames = ffc_frames
-        self.sample_frames = None
+        self.sample_frames = []
 
         if important_frames is not None:
             self.set_sample_frames(important_frames)
@@ -128,7 +128,7 @@ def get_sample_frames(self):
         return self.sample_frames
 
     def remove_sample_frame(self, f):
-        self.important_frams.remove(f)
+        self.important_frames.remove(f)
 
     def get_sample_frame(self, i=0, remove=False):
         if len(self.sample_frames) == 0: