SPRINGFIELD — An independent candidate is asking a full federal appeals court to put him back on the ballot after a three-judge panel knocked him off.Thirteenth Congressional District candidate David M. Gill, a physician from Bloomington, has argued a state law requiring third-party office-seekers to get thousands more signatures than Democrats or Republicans is unconstitutional.And he’s made some headway with that argument. Last month he obtained a preliminary injunction against the law from a federal judge in …