Allows Physics instantiation without Phase, and ability to associate with Phase afterward

[jgostick]

This is pretty subtle, but would help make the code more flexible and forgiving. We've been doing several of these sort of relaxing of conditions lately, so this is part of a larger change. This is stemming from the new PNM IO class, which needs instantiation and object associations to be as flexible as possible (ie. the ability to create things in opposite orders).

[jgostick]

After digging into the code, I see that we already 'try' to allow this, but it doesn't work. The project look-up breaks when no phase is given. I will tag this as a bug.

[jgostick]

We already support this, BUT it breaks if the physics has any models on it. The init should be changed add 'freeze_models' to the settings if phase is None.

[jgostick]

closed by #1674