Add informal Parser interface

Added first informal parser interface, only requiring a method to parse
events and one to parse trackers.

Theoretically we only *require* a method to parse events since, through
their contained activities, they would also come with trackers.
But this would 1) be much more opaque and a lot of work to then extract
the trackers again and 2) leave out trackers which do not yet have any
activities associated with them (i.e. trackers never once accomplished).

We can still turn the informal parser into a formal interface if need
arises: https://realpython.com/python-interface/

src/habitmove/nomiedata.py

@@ -47,7 +47,7 @@ class Event:
     id: str
     start: int
     end: int
-    text: str
+    text: str = ""
     activities: list[Activity] = field(default_factory=lambda: [])
     score: int = 0
     lat: float = 0.0

src/habitmove/parser.py

@@ -0,0 +1,16 @@
+from habitmove.nomiedata import Event, Tracker
+
+
+class Parser:
+    def __init__(self, path: str, filename: str) -> None:
+        """Load in a data set"""
+        self.path = path
+        self.filename = filename
+
+    def extract_trackers(self) -> list[Tracker]:
+        """Extract trackers from the data set"""
+        raise NotImplementedError
+
+    def extract_events(self) -> list[Event]:
+        """Extract events from the data set"""
+        raise NotImplementedError