If we put a scratchpad in `SampledModel`/`IntegratedModel`, we can reuse it to compute `dens` and `dens_diff` and reduce allocations. This should speed up `solve` quite a bit.
If we put a scratchpad in
SampledModel/IntegratedModel, we can reuse it to computedensanddens_diffand reduce allocations.This should speed up
solvequite a bit.