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/opencl/opencljit.py

@@ -28,6 +28,20 @@ def init_globally(device_index=0):
     _global_cl_queue = cl.CommandQueue(_global_cl_ctx)
 
 
+def init_globally_with_context(opencl_ctx, opencl_queue):
+    global _global_cl_ctx
+    global _global_cl_queue
+    _global_cl_ctx = opencl_ctx
+    _global_cl_queue = opencl_queue
+
+
+def clear_global_ctx():
+    global _global_cl_ctx
+    global _global_cl_queue
+    _global_cl_ctx = None
+    _global_cl_queue = None
+
+
 def make_python_function(kernel_function_node, opencl_queue, opencl_ctx, argument_dict=None, custom_backend=None):
     """
     Creates a **OpenCL** kernel function from an abstract syntax tree which