Add incremental design matrix building

[zmbc]

Apologies if this already exists, but I could not find an equivalent of patsy's incr_dbuilder (more info). This would be useful for several reasons, one of them being for parallelizing the actual building across cores.

[matthewwardrop]

Hi @zmbc ! Unfortunately, formulaic does not support incremental building; but in principle it would not be too difficult to implement. The equivalent data structure for DesignInfo in formulaic is the ModelSpec. If you know the categories in advance, you could construct the ModelSpec yourself.

You could also construct your ModelSpec on a subset of the data that is guaranteed to have all categories represented, and then just apply the ModelSpec in parallel. Since ModelSpec is guaranteed to be serializable, this is easy to pass around between process/etc.

The only thing I think that is missing is the original scan of the data to make sure than formulaic detects all categories... is that right?