Make the list of streams easier to modify by the user to add custom streams

[ekluzek]

Right now the list of streams is hardcoded inside the infrastructure that builds the streams.xml for the case. This is great for standard cases, but not flexible when the user wants to do something more custom. This is also a feature that we have in the current MCT implementation and would be good to have a way to do this in CDEPS for NUOPC. We do have cases where users do want/need to do this type of custom work, so facilitating this would be useful. This is important for at least datm -- but making it general for all data models might be useful as well.

For the MCT implementation you can add new streams by modifying the streams variable name in the user_nl_datm for example. And then you can add a user_datm_stream file for the specific stream you are adding. So in CDEPS I envision something like changing the list of streams in a user_* file and then set all the streams variables that are needed for the streams in the user_nl_*_streams file.