Display median game size per system

[massivebird]

This would be a cool statistic!

Prerequisites

• Refactor column logic to be dynamic #11

[massivebird]

Maybe mean game size would be more appropriate. See if the numbers look alright!

The median is providing weird results for the PS1 in particular. Games are usually represented as a .cue file along with one or more .bin files. As far as I can tell, they're both handy to have. The problem is that these files can differ greatly in size: Rainbow Six's 27 bin files range from 160 MB to 1.39 MB.