introduced EntityViewStruct
cut a lot of code not needed anymore
breaking change engines add and remove callback now accept only ref params
breaking change QueryEntityViewsAsArray renamed in to QueryEntityViewsCacheFriendly

# Conflicts:
#	Svelto.ECS/DataStructures/TypeSafeFasterListForECS.cs
#	Svelto.ECS/EnginesRootEngines.cs
#	Svelto.ECS/EnginesRootEntities.cs
#	Svelto.ECS/EnginesRootSubmission.cs
#	Svelto.ECS/Extensions/Unity/GenericEntityDescriptorHolder.cs
#	Svelto.ECS/IEngine.cs
#	Svelto.ECS/IEntityDescriptorHolder.cs
#	Svelto.ECS/MultiEntityViewsEngine.cs
#	Svelto.ECS/Profiler/EngineProfiler.cs
#	Svelto.ECS/SingleEntityViewEngine.cs

Database and ID refactoring

EnttiyStructs can now be built with an initializer

remove some redundant methods

- introduced the concept of global ID, as a combination of groupID and entityID
- only 256 groups and 2^24 entities can be created
- all the entities are now built always in group, when the group is not specified, the "standard" groups is used
- the groupID can be retrieved from the EGID
-

Create LICENSE

add SimpleSubmissionEntityViewScheduler for testing purposes