ADDs methods

[maray]

I've included in my branch (maray) the symbolic perseus code.
There are two things to do here:

1. include in each file the name of Pascal Poupart and explain a little bit what it does.
2. There are two main methods that are needed to implement in this code to fit the abstraction
   i) DD.copy() : I am not confident enough with ADDs to do this correctly
   ii) OP.entropy() : I do not know how to calculate the entropy of a ADD, in any case, entropy is now implemented in CustomVector if it applies.