dg1d: Add limiters.jl

• 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