Merge "Write tests" branch into master

Closes #18 (closed)

src/Dataset.py

@@ -255,7 +255,7 @@ class Dataset:
             logging.error("parameter `colname` is not a string")
             raise ValueError(f"{colname} is not a string")
 
-        return self.dataframe[colname].explode().unique()
+        return self.dataframe[colname].unique()
 
     def get_category_counts(
         self, colname: str, ascending: bool = None