Creater Iterator interface, implemented it in Tuple, created StorageTypes enum and IStorageType and implemented it in Relation and Tuple

* Created enum StorageTypes for types which need to be stored on Page to be stored on disk
* Created IStorageType interface which gets the kind of StorageType it is and implemented it on Relation and Tuple
* Created Iterator interface with Open, Next, Previous, SeekFirst (for reaching front) and Close
* Created BufferManager class in Avion.csproj (the core project)