Dual time integrator - custom field initialization/copy

In dual time integration we copy initial conditions from STATE_N into STATE_NONE for all integrated variables. We need to be able to do this for more than just the integration variables. Temperature is the most important example, where the copy is necessary because T_NONE is used as the initial guess in the temperature solve. The dual time integrator should store a vector of strings as names of fields that are copied from STATE_N to STATE_NONE and it should perform this copy. A function add_n_to_none_field(name) should be added and overloaded for string or Expr::Tag.