Handle `"sc.smooth.spec"` smooths – these are shape constrained B splines used in `scasm()`. These can also use used in tensor products

[gavinsimpson]

The main addition needed is to handle sc.smooth.spec smooths, but these don't need a new method; seemingly the main difference is that these smooths don't have an nCons attribute and the code in spline_values assumed this was present.

• make draw() etc work for these smooths
• check what needs to happen / what mgcv does for these smooths in terms of the overall uncertainty thing