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/autoinit.py

@@ -4,8 +4,13 @@ Automatically initializes OpenCL context using any device.
 Use `pystencils.opencl.{init_globally_with_context,init_globally}` if you want to use a specific device.
 """
 
-from pystencils.opencl import *  # noqa
-from pystencils.opencl.opencljit import *  # noqa
-from pystencils.opencl.opencljit import init_globally
+from pystencils.opencl.opencljit import (
+    clear_global_ctx, init_globally, init_globally_with_context, make_python_function)
 
-init_globally()
+__all__ = ['init_globally', 'init_globally_with_context', 'clear_global_ctx', 'make_python_function']
+
+try:
+    init_globally()
+except Exception as e:
+    import warnings
+    warnings.warn(str(e))