Group Plots

Adds grouped plots by count and group capacity.

 On branch master

 Changes to be committed:
	modified:   Ltt/Display.pm
	modified:   Ltt/Plotting.pm
	modified:   Ltt/Statistics.pm
	modified:   images/hdd_cap_by_size.png
	modified:   images/hdd_cap_by_vendor.png
	modified:   images/hdd_count_by_size.png
	modified:   images/hdd_count_by_vendor.png
	modified:   images/ltt_storage_rankings.png
	modified:   json/constants.json
	modified:   ltt-rankings.pl