Fix MICEDesign tests

[edaub]

The changes to the GP interface that are currently being implemented on a branch break some of the MICE tests. This is because of changes to the default GP that is initialized automatically by the MICE class. This will eventually be fixed to conform to the new interface, at which point the tests should be updated/corrected.

However, the lack of tests and interface changes should not change the correctness of the implementation, just the particular results that are produced from running a model. Thus, the code should remain usable and results can be trusted until this is fixed.