dg1d: Add Bsplines

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.

added feature mesh parameters refactor tests labels

added 1 commit

• f60da217 - fix more docstrings

Compare with previous version

added 32 commits

• f60da217...4abcff88 - 2 commits from branch main
• 4abcff88...f8d18b91 - 20 earlier commits
• a06b2e64 - rm local quadrule, add docstrings
• dacd91fa - add parameter value modal_bspline2 to Mesh.kind; update docstring
• 61da65e4 - update more docstrings
• c253e240 - fix DGElement setup for modal_bspline2
• 1f2b3fbd - add Bspline2 plot script
• 194ac884 - reorganize test
• 1786b516 - rm unit test helper
• e727d94d - test LG quad rule
• 32962cb9 - partition-of-unity test for Bernstein
• f102b525 - fix more docstrings

Compare with previous version

merged

mentioned in commit 909e3d50

mentioned in merge request !215 (merged)

mentioned in commit cb7c756e