feat: find_beat(): bias with f_hint - once we know the beat freq

beat.py

@@ -172,14 +172,15 @@ class RegularBeatFinder:
         # evaluate mean absolute errors for all frequencies
         freqs, freq_errs = self._get_opt_ibi_freq_2(fs, act_ibis, debug_i)
         # bias with f_hint - once we know the beat freq, make it more likely for it to be found everywhere
-        nf, f1, f2 = RegularBeatFinder.num_freqs, RegularBeatFinder.range_f2, RegularBeatFinder.range_f1
-        bias = gauss(
-            nf,
-            (f_hint - f1) / (f2 - f1) * nf,
-            RegularBeatFinder.f_bias_width * nf
-        )
-        freqs_bias = 1.0 / (np.max(bias)+bias)  # make 'f_hint' at most 2x more likely -- (1+bias) if normalized
-        freq_errs *= freqs_bias
+        if f_hint is not None:
+            nf, f1, f2 = RegularBeatFinder.num_freqs, RegularBeatFinder.range_f2, RegularBeatFinder.range_f1
+            bias = gauss(
+                nf,
+                (f_hint - f1) / (f2 - f1) * nf,
+                RegularBeatFinder.f_bias_width * nf
+            )
+            freqs_bias = 1.0 / (np.max(bias)+bias)  # make 'f_hint' at most 2x more likely -- (1+bias) if normalized
+            freq_errs *= freqs_bias
         #
         if debug_fe:
             plt.figure(figsize=(8,2))