Brought back damage per second export again

A revised version of https://github.com/OpenRA/OpenRA/pull/5451 which does the calculation internally as requested to avoid making this useful to spreadsheet users only. It has been designed with more flexibility implementation wise to allow other output formats in the future.

![image](https://cloud.githubusercontent.com/assets/756669/4348962/2d6cc6a2-41a3-11e4-9fc8-2e7798142c44.png)
This is a bit weird - Remove should automatically decrement Length for you.