dg1d: Add Bsplines (!210)

This adds multiple things:
- A `Bspline` module for working with 2nd order Bsplines.
- Extends `DGElement` with the `kind=:modal_bspline2` option.
- Updates docstrings and refactors some tests.
- Adds a `LG` module for Legendre-Gauss quadrature.
- Adds a plotting script `plot/bspline.jl` to tinker with the `Bspline2` approximation.

Also adds a new option to the `Mesh` parameters section:
- `kind = "modal_bspline2"`: Utilize a 2nd order modal Bspline Ansatz for the DG evolution.