Use covariant return types in clone method tempates

src/main/java/org/sablecc/sablecc/node/AAlt.java

src/main/java/org/sablecc/sablecc/node/AAltTransform.java

src/main/java/org/sablecc/sablecc/node/AAst.java

src/main/java/org/sablecc/sablecc/node/AAstAlt.java

src/main/java/org/sablecc/sablecc/node/AAstProd.java

src/main/java/org/sablecc/sablecc/node/ACharBasic.java

src/main/java/org/sablecc/sablecc/node/ACharChar.java

src/main/java/org/sablecc/sablecc/node/AConcat.java

src/main/java/org/sablecc/sablecc/node/ADecChar.java

src/main/java/org/sablecc/sablecc/node/AElem.java

src/main/java/org/sablecc/sablecc/node/AGrammar.java

src/main/java/org/sablecc/sablecc/node/AHelperDef.java

src/main/java/org/sablecc/sablecc/node/AHelpers.java

src/main/java/org/sablecc/sablecc/node/AHexChar.java

src/main/java/org/sablecc/sablecc/node/AIdBasic.java

src/main/java/org/sablecc/sablecc/node/AIgnTokens.java

src/main/java/org/sablecc/sablecc/node/AIntervalSet.java

src/main/java/org/sablecc/sablecc/node/AListTerm.java

src/main/java/org/sablecc/sablecc/node/AMinusBinOp.java

src/main/java/org/sablecc/sablecc/node/ANewListTerm.java