Opencl datahandling

Since the target_dh_refactoring was never merged, we decided to do the refactoring at smaller steps and recover the original OpenCL datahandling PR.

Refactoring to merge target and backend can be done later.

pystencils/datahandling/pyopencl.py

@@ -14,7 +14,8 @@ class PyOpenClArrayHandler:
         if not queue:
             from pystencils.opencl.opencljit import get_global_cl_queue
             queue = get_global_cl_queue()
-        assert queue, "OpenCL queue missing"
+        assert queue, "OpenCL queue missing!\n" \
+            "Use `import pystencils.opencl.autoinit` if you want it to be automatically created"
         self.queue = queue
 
     def zeros(self, shape, dtype=np.float64, order='C'):