slow operations in overlay()

[sammlapp]

A couple parts of the overla() function in preprocess.actions can be very slow with large overlay_df dataframes:

dropping one index from large df is slow:

opensoundscape/opensoundscape/preprocess/actions.py

Lines 743 to 744 in f50022b

	warnings.warn(f"Invalid overlay sample: {overlay_path}")
	if len(overlay_df) < 1:

.loc[] with multi-index is slow, reset_index for fast integer based index lookup

opensoundscape/opensoundscape/preprocess/actions.py

Line 697 in f50022b

overlay_sample = AudioSample.from_series(overlay_df.loc[overlay_path])