(that's a massive rewrite!)
This change might seem random, but it, in fact, simplifies usage A LOT.
Now, `Scalar` type is **determined** by model, which is what should
happen in the first place.
- Now using `generic_array_storage` v0.1.1 (Conv trait drastically
changed).
- `FitModel::ParamCount` type now has a less strict type bound, allowing
use of `typenum::Const` and even `nalgebra::Const`, if you feel like
it.
- Removed `Params` argument from `ConstOptimizationProblem` and
`DynOptimizationProblem`.
