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 gives way more significant figures than we need.  There doesn't look to be a simple way to set the output precision here, so you could define this as `string` and then format the input yourself.