fix: adding new line each prose subpart gets added to final subpart

Signed-off-by: Alejandro Jose Leiva Palomo <[email protected]>

trestle/core/control_interface.py

@@ -120,7 +120,7 @@ def _gap_join(a_str: str, b_str: str) -> str:
         b_clean = b_str.strip()
         if not b_clean:
             return a_clean
-        gap = '\n' if a_clean else ''
+        gap = '\n\n' if a_clean else ''
         return a_clean + gap + b_clean
 
     @staticmethod