Wrapper for midband frequency of standard filter

src/filter/octave.rs

@@ -567,6 +567,10 @@ impl StandardFilterDescriptor {
         Ok(Self::genFilterSetForRange(b, fmin, fmax, append_overall)?)
     }
 
+    #[pyo3(name="fm")]
+    fn fm_py(&self) -> Option<Flt> {
+        self.fm()
+    }
 }
 
 #[cfg(test)]