Add sparse versions of the Model/Linear classes.

We could make fancy interfaces here, but Linear is really tied to Model,
and I'd like to keep the upstream sources as pristine.

Change-Id: Ie69622ec6356b9b4ff679abf32806fc9c4a10293
Use quick sets, since bounds are already checked.

Change-Id: I073ca51219bad57ea2c1a23ee625c5f80961b6f8
Merge pull request #1 from GridLine/sparsemodel

Sparsemodel