Dycore needs to read in constituent data onto the dyn grid

### What is the feature/what would you like to discuss?

Instead of the current implementation where the initial data for constituents is read in in phys_run2, this data should be read in by the dycore. As part of this, there will be a new metadata field "default_value" for when the constituent is not found on the file.

### Is there anyone in particular you want to be part of this conversation?

_No response_

### Will this change (regression test) answers?

No

### Will you be implementing this enhancement yourself?

Yes