Add Maximum Likelihood Expectation Maximation algorithm

This adds two simple implementations of MLEM and the ordered subsets version of it.

@LasseR I'm not 100% sure if these are correct and how we'd use them, however for a simple dataset (one we also have an example), this works as expected and give reasonable results. I'd await your initial feedback for the review, but I think it's still nice to have this lying around :-)