add private module to create model object for acoustics api

opendbm/api_lib/verbal_acoustics/_pitch_frequency.py

@@ -0,0 +1,14 @@
+import pandas as pd
+
+from opendbm.api_lib.model import AudioModel
+from opendbm.dbm_lib.dbm_features.raw_features.audio.pitch_freq import run_pitch
+
+
+class PitchFrequency(AudioModel):
+    def __init__(self):
+        super().__init__()
+        self._params = ["aco_ff"]
+
+    @AudioModel.prep_func
+    def _fit_transform(self, path, **kwargs):
+        return run_pitch(path, ".", self.r_config, save=False)