refactor(analyzer): redefine *_analyzer into steps

- update imports and names
- define step as a protocol
- extract analyzer function from classes

analyzer/libretime_analyzer/steps/step.py

@@ -1,9 +1,7 @@
-# TODO: use an abstract base class (ie. import from abc ...) once we have python >=3.3 that supports @staticmethod with @abstractmethod
+from typing import Any, Dict, Protocol
 
 
-class Analyzer:
-    """Abstract base class for all "analyzers"."""
-
+class Step(Protocol):
     @staticmethod
-    def analyze(filename, metadata):
-        raise NotImplementedError
+    def __call__(filename: str, metadata: Dict[str, Any]):
+        ...