[alexfikl]

This is another chunk of #195. It adds the syzygy-based factorization of IntG systems that should reduce the number of FMMs.

TODO:

• Actually understand the algorithm.
  • It seems like sometimes it can fail. Would be nice to figure that out.
• Add a bunch of tests.
• See if anything can be moved out to sumpy or pymbolic.

ROADMAP:

• Rewrite an expression using a common base kernel: Implement kernel rewriting sumpy#279.
• (This) Factorize a system of IntGs: Reduce FMMs for PDE systems #338.
• Integrate rewrite_using_base_kernel into pytential.
• Integrate reduce_fmms with the rewrite.
• Merge the result into a new expression with less FMMs.
• Profit!

cc: @isuruf
xref: #195