Add attribute and resetSource() to IndirectAttribute

# Conflicts:
#	Sources/OpenGraphCxx/Attribute/OGAttribute.cpp
#	Sources/OpenGraphCxx/include/OpenGraph/OGAttribute.h

Author: jcmosc

Sources/OpenAttributeGraph/Attribute/Indirect/IndirectAttribute.swift

@@ -15,11 +15,19 @@ public struct IndirectAttribute<Value> {
         identifier = source.identifier.createIndirect(size: UInt64(MemoryLayout<Value>.size))
     }
 
+    public var attribute: Attribute<Value> {
+        Attribute(identifier: identifier)
+    }
+    
     public var source: Attribute<Value> {
         get { Attribute(identifier: identifier.source) }
         nonmutating set { identifier.source = newValue.identifier }
     }
 
+    public func resetSource() {
+        __OAGGraphResetIndirectAttribute(identifier, false)
+    }
+    
     public var dependency: AnyAttribute? {
         get {
             let dependency = identifier._indirectDependency

Sources/OpenAttributeGraphCxx/Attribute/OAGAttribute.cpp

@@ -103,6 +103,10 @@ void OAGGraphSetIndirectAttribute(OAGAttribute attribute1, OAGAttribute attribut
     // TODO
 }
 
+void OAGGraphResetIndirectAttribute(OAGAttribute attribute, bool non_nil) {
+    // TODO
+}
+
 OAGAttribute OAGGraphCreateAttribute(long index, const void *body, const void * value) {
     // TODO
     return OAGAttributeNil;

Sources/OpenAttributeGraphCxx/include/OpenAttributeGraph/OAGAttribute.h

@@ -89,6 +89,10 @@ OAG_EXPORT
 OAG_REFINED_FOR_SWIFT
 void OAGGraphSetIndirectAttribute(OAGAttribute attribute1, OAGAttribute attribute2) OAG_SWIFT_NAME(setter:OAGAttribute.source(self:_:));
 
+OAG_EXPORT
+OAG_REFINED_FOR_SWIFT
+void OAGGraphResetIndirectAttribute(OAGAttribute attribute, bool non_nil);
+
 OAG_EXPORT
 OAG_REFINED_FOR_SWIFT
 OAGAttribute OAGGraphCreateAttribute(long index, const void *body, const void * _Nullable value);