Extend KernelAnalysis to mark buffers of read-only fields as const

The buffers of fields that are never written to in a kernel should get a const element type. This MR extends the KernelAnalysis pass to create const buffers accordingly.

Final piece of the puzzle; closes #74 (closed) .

Extends test_domain_kernels to include fp32, and fixes a bug in the cupy JIT where scalar kernel arguments were not being cast to the correct data type.