dg1d: Add limiters.jl (!221)

- Collect most of the limiter implementations into `limiters.jl` (there still remain bits in `ScalarEq`)
- Add new limiter `limit_slope_preserve_continuity!` that is tailored towards the Bspline approximation (although its not working well)
- Add new limiter `limit_slope_convex_hull!` that is tailored towards the Bspline approximation
- Add new reftests `burgers_sine_bspline1_limit_slope_convex_hull` and ``burgers_sine_bspline2_limit_slope_convex_hull`
- Fix Bspline integration