types: Set group_id as mandatory attr for Transcript

Author: lisavader

src/geneml/types.py

@@ -37,7 +37,7 @@ class Transcript:
     events: tuple[GeneEvent, ...]
     score: float
     exons: tuple[Exon, ...]
-    group_id: int = -1
+    group_id: int
     transcript_id: str = ""
 
     def __post_init__(self):