fixed time weighted mean bug

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rowers/dataprep.py

@@ -1634,6 +1634,7 @@ def testdata(time, distance, pace, spm):
 def getrowdata_db(id=0, doclean=False, convertnewtons=True):
     data = read_df_sql(id)
     data['x_right'] = data['x_right'] / 1.0e6
+    data['deltat'] = data['time'].diff()
 
     if data.empty:
         rowdata, row = getrowdata(id=id)