Implemented initial DLPak design

Implemented DLPak design with zip storage, DLPak consists of two files,
a manifest.json which provides details on the signals, time of recording
and currently blank comment field and data.csv which stores all the
signals as csv with the nanosecond timestamp and indexed against the
signal id

src/dynalab_core/buffer.py

@@ -4,6 +4,7 @@
 
 from collections import defaultdict
 import csv
+import io
 from pathlib import Path
 from threading import Lock
 from uuid import UUID
@@ -22,11 +23,15 @@ class ValueBuffer:
         with self._lock:
             self._samples.clear()
 
+    def get_signal_ids(self) -> list[UUID]:
+        with self._lock:
+            return list({signal_id for _, signal_id, _ in self._samples})
+
     def __len__(self) -> int:
         with self._lock:
             return len(self._samples)
 
-    def export_csv(self, path: str | Path) -> None:
+    def export_csv(self) -> str:
         with self._lock:
             samples = list(self._samples)
 
@@ -39,22 +44,25 @@ class ValueBuffer:
 
         ordered_signal_ids = sorted(signal_ids, key=str)
 
-        with Path(path).open(mode="w", newline="", encoding="utf-8") as file:
-            writer = csv.writer(file)
+        output = io.StringIO(newline="")
+
+        writer = csv.writer(output)
+
+        writer.writerow(
+            ["timestamp_ns", *(str(signal_id) for signal_id in ordered_signal_ids)]
+        )
+
+        for timestamp_ns in sorted(rows):
+            timestamp_values = rows[timestamp_ns]
 
             writer.writerow(
-                ["timestamp_ns", *(str(signal_id) for signal_id in ordered_signal_ids)]
+                [
+                    timestamp_ns,
+                    *(
+                        timestamp_values.get(signal_id, "")
+                        for signal_id in ordered_signal_ids
+                    ),
+                ]
             )
 
-            for timestamp_ns in sorted(rows):
-                timestamp_values = rows[timestamp_ns]
-
-                writer.writerow(
-                    [
-                        timestamp_ns,
-                        *(
-                            timestamp_values.get(signal_id, "")
-                            for signal_id in ordered_signal_ids
-                        ),
-                    ]
-                )
+        return output.getvalue()