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

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.

added feature refactor tests labels

added 22 commits

• 90b27166 - 1 commit from branch main
• 90b27166...46a16ba9 - 11 earlier commits
• 4ce7c44d - todos
• 9e6ab88c - more updates to docstrings of GLGL and LGL modules
• acf1047c - add more numutils
• 381968ee - add helpers to bernstein module
• 93710555 - add Legendre submodule
• e705fa7a - add more tests
• 953b9a47 - update SpectralElement
• 8c815895 - rename SpectralElement to DGElement
• cbf6c945 - rename files
• 6b0b2b21 - fix tests

Compare with previous version

merged

mentioned in commit 78408400