Refactor `criterion` class

[Giannis1993]

After talking to @artofscience, we agreed upon making the following changes to the convergence criteria class:

1. Rename converged object to criterion tp make it self-explanatory.
2. Add a criterion.value attribute in order to access its value without recalculating it.
3. Return an array of values if multiple criteria are considered at the same time.

[MaxvdKolk]

For the criterion.value you can use the done attribute implemented by the Criterion class. To return a list of values we need to walk the tree of Criterion classes contained in the Criteria when using the & | ~ operators. I can try to add that feature.