dg1d: Rename `SpectralElement` to `DGElement` and generalize its purpose...

dg1d: Rename `SpectralElement` to `DGElement` and generalize its purpose (!206)

Generalization should allow it represent DG operators that are not necessarily based on nodal approximations via Lagrange polynomials.

Atm we implement the `nodal_bernstein` version, which is actually pseudo-nodal and not yet tested. Will be done in a follow-up PR.

Along the way we also implement some operators for Bernstein and Legendre bases.