GPU AtPoints Assembly

[jeremylt]

Following up from #1799

We need to support this assembly for GPU as well. At least initially my plan is to only support this assembly for /gpu/*/gen.

[jeremylt]

Diagonal assembly here: #1833

@zatkins-dev this file has the packing ordering for full assembly: https://github.com/CEED/libCEED/blob/main/include/ceed/jit-source/cuda/cuda-ref-operator-assemble.h