[#174] Replace a SQL query with the equivalent spark expression

This prevents a possible SQL injection error by setting alpha_threshold to
something weird.  It's also a bit easier to read and work with in my
experience. It's more composable since you can build up the expression instead
of having to write all of the SQL at once.

hlink/linking/core/threshold.py

@@ -7,7 +7,7 @@
 
 from pyspark.sql import DataFrame
 from pyspark.sql.window import Window
-from pyspark.sql.functions import rank, lead
+from pyspark.sql.functions import col, lead, rank, when
 
 
 def get_threshold_ratio(
@@ -79,10 +79,8 @@ def predict_using_thresholds(
 
 
 def _apply_alpha_threshold(pred_df: DataFrame, alpha_threshold: float) -> DataFrame:
-    return pred_df.selectExpr(
-        "*",
-        f"CASE WHEN probability >= {alpha_threshold} THEN 1 ELSE 0 END AS prediction",
-    )
+    prediction = when(col("probability") >= alpha_threshold, 1).otherwise(0)
+    return pred_df.withColumn("prediction", prediction)
 
 
 def _apply_threshold_ratio(