Added pool.Reset() which clears groups and resets creationIndex
Added pool.Reset() which clears groups and resets creationIndex

robinryf/Entitas-CSharp,sschmid/Entitas-CSharp,sschmid/Entitas-CSharp,robinryf/Entitas-CSharp,vkuskov/Entitas-CSharp,vkuskov/Entitas-CSharp