Added used_length and remaining_length API fields

They're calculated from the weights

Resolves #37

tests_integration/tests/conftest.py

@@ -1,5 +1,6 @@
 """Test fixtures for integration tests."""
 
+import math
 import os
 import time
 from enum import Enum
@@ -92,3 +93,19 @@ def random_filament(random_vendor: dict[str, Any]):
 
     # Delete filament
     httpx.delete(f"{URL}/api/v1/filament/{filament['id']}").raise_for_status()
+
+
+def length_from_weight(*, weight: float, diameter: float, density: float) -> float:
+    """Calculate the length of a piece of filament.
+
+    Args:
+        weight (float): Filament weight in g
+        diameter (float): Filament diameter in mm
+        density (float): Density of filament material in g/cm3
+
+    Returns:
+        float: Length in mm
+    """
+    volume_cm3 = weight / density
+    volume_mm3 = volume_cm3 * 1000
+    return volume_mm3 / (math.pi * (diameter / 2) ** 2)