dg1d: Fix convention for Bernstein polynomials

We now establish the convention that all basis functions are indexed by one.

Also had to update the modal_bernstein_advection_sine reftest data, because !201 (merged) skipped running CI on main and me cancelling some other CI runs.

added 1d ScalarEq refactor update reftest labels

Indeed, tests failed on !202 (merged) due to this, see https://git.tpi.uni-jena.de/dg/dg1d.jl/-/jobs/15688

enabled an automatic merge when the pipeline for ca8fc782 succeeds

merged

mentioned in commit cd4a82d6