Tree nodes for symbolic expression trees are derived from Item

[gkronber]

In a GP runs we construct and dispose millions of tree nodes so they are a relevant factor for memory consumption and performance. It should be investigated if it is necessary that tree nodes derive from Item. If we don't derive from Item we might save a significant amount of memory and improve performance.

This ticket should track efforts concerning the reduction of memory consumption for tree nodes.

[mkommend]

Unfortunately SymbolicExpressionTreeNodes are not derived from Item. Instead they just inherit from DeepCloneable and implement the ISymbolicExpressionTreeNode interface. Hence, no memory can be saved and the performance cannot be improved.