Allow node attributes to be typed like node values

[mbreissi]

Currently, attributes are not typed. That is not an issue for attributes that are set programmatically through an SDK. However, because DGLux allows for a user to manually set attribute values, it means an unconscientious user could set the attribute to a value incompatible with its intended type . Therefore, handlers that deal with set operations on attributes must implement type error checking to account for DGLux which would otherwise not be necessary (i.e., if attributes had a type which DGLux could "read" and therefore restrict the input).