Accessor::Sparse needs various improvements

[agrittmsft]

1. Accessor.sparse member should be a unique_ptr<Sparse> instead of just Sparse so that a null value can be represented properly
2. Accessor::Sparse should inherit from glTFProperty
3. Accessor::Sparse.indices and Accessor::Sparse.values should inherit from glTFProperty

Currently any extensions or extras stored on these nodes will be lost upon import.