Use well to name mapping when copying the old well state to the new

With the introduction of shut wells the same ordering in the well states
can no longer be assumed. The well names is instead used to map the old
well state to the new. Also the partial_copying is moved into the
initialization.

This PR is a stripped version of a patch a got from @dr-robertk 
So questions may be addressed to him as well. 

Tested on SPE1, SPE3, SPE9 and Norne. 

Do not change the SPEs and is necessary for Norne, but not sufficient. More PRs are on its way. 
I think this is a good time to add a little more doxygen docs for this, it should tell me what is expected from prevState (can it be a default-initialised object, or must it be initialised for example).