add more canonical splice sites

[diekhans]

STAR supports these

	5.	intron motif – Splice donor/acceptor motif:
	•	0 = non-canonical
	•	1 = GT/AG
	•	2 = CT/AC
	•	3 = GC/AG
	•	4 = CT/GC
	•	5 = AT/AC
	•	6 = GT/AT