Fix infix 1

[mmatera]

Once the arithmetic format is fixed, then we can fix how Infix[...] expressions are formatted.
Now,

• Operator (the second argument) is always a string (or at least, Lists are not processed in an special way)
• associativity is controlled by the group property of the elements of Infix, and not from outside, according to WL.

For example, Infix[{a, Infix[{b,c},"#",300, Left]}, "@", 300, Right] means that the inner Infix is embraced by parenthesis (because Left associative are embraced except if they are in the first position).