Add a Variable for TIME

[kabdelhak]

Most of the new Backend is based on the fact that component references have a pointer to their variable representatives. Currently we always need to check if the cref is TIME because that does not have a pointer to a variable.

Easy solution: create a variable for TIME during NBackendDAE.lower() and add a pointer to it to all TIME crefs. We do not really need to treat it differently in most cases. And it would clean up the code and make it easier to maintain because there will always be a variable pointer to a cref and a cref to a variable pointer.