Task 3: Allow input of variables to CalculatorBrain

[petervanhoef]

Add the capability to your CalculatorBrain to allow the input of variables. Do so by
implementing the following API in your CalculatorBrain ...
func setOperand(variable named: String)
This must do exactly what you would imagine it would: it inputs a “variable” as the operand (e.g. setOperand(variable: “x”) would input a variable named x). Setting the operand to x and then performing the operation cos would mean cos(x) is in your CalculatorBrain.

[petervanhoef]

This task will be implemented together with task 4 (see #2), as it can't be tested standalone.