Create a collection histogram used for image cleaning process

[mejackreed]

c = a.merge(b){ |k, v1, v2| v1.to_i + v2.to_i }
# and then
irb(main):033:0> a["(0,0,0)"]
=> "3"
irb(main):034:0> b["(0,0,0)"]
=> "13"
irb(main):035:0> c["(0,0,0)"]
=> 16