Loading in Equilibria saved before constraint-toroidal-current PR results in errors due to nonexistent eq.current property

Per title, if loading in an equiilbrium solved before the toroidal current constraint PR, it won't have a `self.current` property, and so will result in an error if you try to compute anything from it

can easily fix by having a `_set_up()`  function for Configuration to check if the loaded eq has a `current` property, and if it does not it sets it to `None`