WIP: SBP-DG Interface

Attempts to implement the SBP-DG coupling methods of Kozdon and Wilcox, starting with modification of the advection symbolic operator.

• ⚠ Needs meshmode!73 (closed) to be merged
• ⚠ Point requirements.txt back to main before merging

K + W Paper: https://arxiv.org/pdf/1410.5746.pdf