How to design a switched mode battery discharger
The main difference between charging and discharging is that during discharge you want to waste power. Therefore no matter what technique you use, you will need something to turn the power into heat. 

The simplest way to do this is just switch in resistors in parallel until you get the desired current. However if the resistance is fixed you will need to log current or voltage periodically during the discharge. For finer adjustment you could PWM one of the resistors, perhaps with feedback to regulate the average current. Smoothing may not be required because the battery itself acts as a filter.     

Another way to get constant discharge current is with a linear regulator. Set the output voltage to just below the minimum battery voltage minus regulator dropout voltage, and put resistance across the output to draw the desired current. The regulator only has to 'burn off' the voltage difference, so the load resistor(s) will absorb most of the power.         

On the other hand you might want to reproduce operating conditions for a device that draws constant power (eg. a switching DC/DC converter). In that case you could simply use an off-the-shelf DC/DC converter with load resistors on the output.  

To get constant input current to a switching converter you need to monitor the input current and use it to control the PWM ratio. A boost-mode converter draws continuous current, but it is not necessarily smooth