* update Swift inteface for more fine grained control
- move instance method to static method which does not require spine cpp pointer
- implement equality and hashing based on cpp pointer so that it can be stored in collection and compare it
- expose Atlas image count property so that the user can load the atlas resource lazily
- User can now Create Atlas by using Altas static function, and fetch whole resource path by iterating count of atlas page count
* [iOS] fix wrong y-axis alignment
* implement safe bounded data Access and cancellation supporting URLSession downloadTask
* declare specific SpineError type
declaring conformance to Error on String is discouraged, and Creating own Error type is recommended
* use explicit Error initializing rather than casing syntax (apply requested change)
