dg1d: Add Bsplines1 (!215)

Continuation of !210

This adds a dispatch type `Bspline1` similar to `Bspline2`. Based on this we implement the necessary method to be able to use it in `DGElement`.

I also fixed a subtle bug in the quadrature rules I used in some of the `Bspline` methods.
That is, I accidentally used a LG rule with `Npts=2`, which corresponds to a `N=Npts-1=1` order rule that is exact for polynomials up to `2*N=2` order.
But for `Bspline2` we encounter fourth order polynomials, so we should be using `Npts=3`.

Also adds a new option to the `Mesh` parameters section:

* `kind = "modal_bspline1"`: Utilize a 1st order modal Bspline Ansatz for the DG evolution.