WIP: Add check for missing arguments

• The issues related to numpy input should be resolved by the changes to PyOpenCLExecutionWrapperGenerator in the first commit. merge in new MR
• I've taken a ham-fisted approach to make sure input arguments are required and added KernelExecutorBase.check_for_required_arguments and call it from PyOpenCLKernelExecutor.__call__. (Let me know if you'd prefer a different approach.)
  • This breaks test_indexof and test_indexof_vec for now.
• As an extra (simple) precaution, KernelExecutorBase.generate_arg_setup now only allocates arrays if they are only written to but not read. I haven't added any error handling at that level (for missing input arrays), but I figured that (to start) having an exception arise at some point because of an unallocated array was better than silently allowing incorrect invocations.
• The demo code from #196 (closed) has been added as tests.

Closes #196 (closed).