[CppAD] Change to VecAD specifications

[Brad Bell]

There seems to be some problem with the interaction between the VecAD class
    http://www.coin-or.org/CppAD/Doc/vecad.htm
and optimizing compilers. For example g++ with the -O2 option.

I am considering changing the VecAD class so that the indexing operations
that get recorded on the tape have a different syntax. For example:
    V[index],  might be changed to, V.get(index)
    V[index] = value, might be changed to, V.set(index, value)

Would this cause problems for any of the current users of CppAD ?