new: support for negative prop index

src/scgenerator/spectra.py

@@ -157,6 +157,8 @@ class Propagation:
             return self._load_slice(key)
         if isinstance(key, (float, np.floating)):
             key = math.argclosest(self.parameters.compute("z_targets"), key)
+        elif key < 0:
+            key = len(self) + key
         array = self.io.load_spectrum(key)
         return Spectrum(array, self.parameters)