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.