Some cleanup to the pystencils runtime headers

• Remove ancient PyStencilsField.h
• move half-precision and HIP-specific integer typedefs to pystencils_runtime/*.h directory
• remove INFINITY macros since they cause redefinition errors with nvcc and hipcc

Over time, we should move all runtime headers to the pystencils_runtime folder such that the associated #includes can be properly qualified.

At the moment, the INFINITY macros are used nowhere in the code base. For their reintroduction in !438 (merged), we will need to set up a more stable implementation.

src/pystencils/backend/platforms/cuda.py

@@ -188,7 +188,7 @@ class CudaPlatform(GenericGpu):
 
     @property
     def required_headers(self) -> set[str]:
-        return {'"gpu_defines.h"'}
+        return {'"pystencils_runtime/hip.h"'}  # TODO: move to HipPlatform once it is introduced
 
     def materialize_iteration_space(
         self, body: PsBlock, ispace: IterationSpace