Fix IFuture.GetValue

Make it wait if not done.
Add license clause to files
Refactored IFuture<T>

Public type changed to Future<T>, moved to its own unit.
Added max, min, pow built-in functions

Not tested with interpreter yet.
Add div and or operators to expr tree, improve built-in function handling

Bodies of built-in functions will have an empty (nil) expression node
implementation. Added IsBuiltIn property to NaryFunc.
Added support for user-defined functions in compute expressions