Lazy LSQ System

[cortner]

I spent a week hand-tuning a LSQ fit. To do so, I manually managed what IPFitting used to provide within LsqDB. I'd like us to re-introduce such a functionality, but maybe go a step further and make this a lazy datastructure that assembles the design matrix "as needed".

The "standard usage" would remain mostly unaffected by this I think or it could even become an option that need not be used by most users.

For now this is just a note - we can discuss it before doing anything.