Explicitly disallow multiple parse_legacy calls.

These didn't work anyway, as node names are lost after the parse is
completed.

Closes #247.

src/finesse/script/__init__.py

@@ -66,6 +66,14 @@ def parse_legacy(text, model=None, ignored_blocks=None):
     :class:`.Model`
         The parsed model.
     """
+    if model:
+        # Newly-created models contain an fsig, so we need to account for that
+        if len(model.elements) > 1:
+            raise ValueError(
+                "Legacy parsing of extra commands with an existing model is unsupported. "
+                "Please switch to the new syntax, or only call 'parse_legacy' on a complete kat"
+                "file."
+            )
     parser = LegacyKatParser()
     return parser.parse(text, model=model, ignored_blocks=ignored_blocks)
 
@@ -90,6 +98,14 @@ def parse_legacy_file(path, model=None, ignored_blocks=None):
     :class:`.Model`
         The parsed model.
     """
+    if model:
+        # Newly-created models contain an fsig, so we need to account for that
+        if len(model.elements) > 1:
+            raise ValueError(
+                "Legacy parsing of extra commands with an existing model is unsupported. "
+                "Please switch to the new syntax, or only call 'parse_legacy' on a complete kat"
+                "file."
+            )
     parser = LegacyKatParser()
     return parser.parse_file(path, model=model, ignored_blocks=ignored_blocks)