feat(mod) Add frame extractor method

Adds a method to the corpus class that uses ffmpeg in order to extract
frame images from a given movie file in the corpus.

itten/corpus.py

 from pathlib import Path
+from subprocess import Popen, PIPE, CalledProcessError, run
 from cv2 import imread, imwrite
 import numpy as np
 from .movie import Movie
@@ -100,6 +101,8 @@ class Corpus(object):
         """
         tasks = self._extraction_tasks()
 
+        # TODO Extractors implementieren
+
         return tasks
 
     def _extraction_tasks(self):
@@ -167,10 +170,50 @@ class Corpus(object):
 
         return tasks
 
-    def _extract_frames(self):
+    # TODO Die _extract Methoden gehören eigentlich zur Movie Klasse
+    def _extract_frames(self, movie='in_the_loop', fps=4, scale=240):
+        """Extract frames from movie files
+
+        The implementation uses ffmpeg in order to extract frames from a movie
+        file in a Matroska container.
+        
+        Keyword Arguments:
+            movie {str} -- the filename of the movie file without the file
+                extension.add() (default: {'in_the_loop'})
+            fps {int} -- how many frames per second are extracted from the
+                movie file.add() (default: {4})
+            scale {int} -- the resulting size of the movie measured in vertical
+                hight (default: {240})
+        """
+        # Create frame images directory
+        Path(self.basedir / movie / 'frames' / (str(scale) + 'p30')).mkdir(
+             parents=True, exist_ok=True)
+
+        # build shell string for popen
+        i = '"' + str(self.basedir) + '/' + movie + '/movie/' +\
+            movie + '.mkv' + '"'
+        vf = '"scale=iw*sar:ih,scale=-1:' + str(scale) + ',fps=4"'
+        o = '"' + str(self.basedir) + '/' + movie + '/frames/' +\
+            str(scale) + 'p30/' + movie + '_' + '%' + '05d.png' + '"'
+        cmd = 'ffmpeg ' + '-i ' + i + ' -vf ' + vf + ' ' + o
+        # -vf 'fps=1' = 1 pro Sekunde; `fps=1` 2 pro Sekunde; `fps=1/60` alle
+        # 60 Sekunden 1 Bild
+        # %05d Namingpattern für die Ausgabe Dateien
+        print('Extracting frames from {0}'.format(movie))
+        # Spawn ffmpeg process
+        response = run(cmd, shell=True, stdout=PIPE, stderr=PIPE,
+                       encoding='utf-8')
+
+        # Evaluate the result
+        if response.returncode != 0:
+            print(response.stderr)
+        else:
+            print('Finished!')
+
+    def _extract_data(self):
         pass
 
-    def _extract_contrast(self):
+    def _extract_visuals(self):
         pass
 
     def tableau(self, mode='contrast',