kranc2.peg: Identify lower and upper indices in the parser

Previously this logic was in Kranc, where it has to be more complicated.

Auxiliary/Grammars/kranc2.peg

@@ -3,7 +3,9 @@ int = [0-9]+
 end = ([ \t]+|#.*)*\r*\n
 name = [A-Za-z][A-Za-z0-9]*
 uname = [A-Za-z][A-Za-z0-9_]*
-indices = ([_^]({index_symbol}|\({index_expr}(,{index_expr})*\))+)*
+lower_index = ({index_symbol}|\({index_expr}(,{index_expr})*\))
+upper_index = ({index_symbol}|\({index_expr}(,{index_expr})*\))
+indices = (\_{lower_index}+|\^{upper_index}+)*
 index_symbol = [a-zA-Z]'*|[0-9]
 index_expr = {index_symbol}([+-]{int}|)
 tensor = {name}{indices}