feat: add FeatureExtractor interface

or at least what passes for an interface in python

pipeline/feature_extractors.py

@@ -0,0 +1,15 @@
+from abc import ABC
+import logging
+logger = logging.getLogger(__name__)
+
+class FeatureExtractor(ABC):
+    """Feature extractor interface."""
+    # TODO: #API -- decide if .features will be a member variable
+    def setup(self):
+        pass
+
+    def run(self):
+        pass
+
+    def teardown(self):
+        pass