Update ppg_quality.py

Addressing issue #1043
Change 'templatematch' implementation to use interpolation method 'previous' rather than 'quadratic'. This aligns it with the 'disimilarity' implementation and is a better representation of the beat-by-beat nature of the quality measure.

neurokit2/ppg/ppg_quality.py

@@ -138,7 +138,7 @@ def _ppg_quality_templatematch(ppg_cleaned, ppg_pw_peaks=None, sampling_rate=100
 
     # Interpolate beat-by-beat CCs
     quality = signal_interpolate(
-        ppg_pw_peaks[0:-1], cc, x_new=np.arange(len(ppg_cleaned)), method="quadratic"
+        ppg_pw_peaks[0:-1], cc, x_new=np.arange(len(ppg_cleaned)), method="previous"
     )
 
     return quality
@@ -193,4 +193,4 @@ def _ppg_quality_disimilarity(ppg_cleaned, ppg_pw_peaks=None, sampling_rate=1000
         ppg_pw_peaks[0:-1], dis, x_new=np.arange(len(ppg_cleaned)), method="previous"
     )
 
-    return quality
+    return quality