Add performance tests to Xcode unit test target.
Increase the performance test sample size.
Add RandomnessDelegate

The randomness delegate wraps random number generation in a class method, so that randomness can be mocked out during testing.
Move the ising grid fixture to the shared text fixtures header.
Provide binary events directly from the concrete randomness delegate.
Add mock randomness delegate.
Add a test fixture for testing operations that manipulate grids.
Use dynamic polymorphism for RandomnessDelegate.
Write interface and failing test suite for the metropolis algorithm.