Better interface for mfs option
The mfs option of the model keyword, that enables an additional optimization in the block decomposition, has an unfriendly interface. It currently accepts an integer, while it would be more natural to use keywords (the challenge is to choose keywords that reflect the natural ordering of the possible values).
The old integer values would still have to be kept for backward compatibility.
From an implementation point of view, the different values should be stored as an enum class in the preprocessor.