Binary function call

[bertiqwerty]

Addresses issues #51 and #41

All binary operators can be used either like a op b or like op(a, b). Thereby, the latter will be interpreted as ((a) op (b)). For instance

atan2(y * 2, 1 / x) * 2

and

((y * 2) atan2 (1 / x)) * 2

are equivalent. As a side-product, atan2 has been added to the list of default-operators.