This PR fixes some copy and move bugs for attributes. Previously, the graph pointer stored in some attribute classes did not change when the graph was copied, and data stored in attributes was not copied.

Changes:

• Attribute classes are now a seperate header (taken from Graph: Seperate core functions from common API #1222)
• Sanity checks are now debug-mode only (solves Attributes: move sanity check to Debug-Build #1248)
• There is now a clear 1:1 relation between Graph and AttributeMap
• AttributeMap cannot be copied implicitly; a new graph pointer is required
• removed the graph pointer from AttributeStorage; validity checks are now done in Attribute instead
• added explicit copy assignment and constructor to Graph to use the new AttributeMap copy constructor
• fix some details in the graphTools conversion code
• Graph move operators will update the pointer in AttributeMap; Attribute objects are invalid after a move.