Some right-hand operators in infix notation are missing

[marcogario]

When writing 1 + x python tries the __radd_ operator, and here we can perform the casting of left operand. Some right-hand operators are currently not implemented in Fnode. These include: __rgt__, __rge__, __rdiv__ .

Note: we need to unify the procedure to properly cast the left-handside, see how we do __rsub__ as a special case, and how __radd__ inverts the operands.

This requires revising the tests for infix notation, and make sure that we have one test for each right-hand version of the operators.