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/kernelcreation.py

-from itertools import combinations
+import functools
+import itertools
 from types import MappingProxyType
 
 import sympy as sp
@@ -12,7 +13,6 @@ from pystencils.simp.assignment_collection import AssignmentCollection
 from pystencils.stencil import direction_string_to_offset, inverse_direction_string
 from pystencils.transformations import (
     loop_blocking, move_constants_before_loop, remove_conditionals_in_staggered_kernel)
-import functools
 
 
 def create_kernel(assignments,
@@ -143,7 +143,9 @@ def create_indexed_kernel(assignments,
                           cpu_openmp=True,
                           gpu_indexing='block',
                           gpu_indexing_params=MappingProxyType({}),
-                          use_textures_for_interpolation=True):
+                          use_textures_for_interpolation=True,
+                          opencl_queue=None,
+                          opencl_ctx=None):
     """
     Similar to :func:`create_kernel`, but here not all cells of a field are updated but only cells with
     coordinates which are stored in an index field. This traversal method can e.g. be used for boundary handling.
@@ -193,7 +195,7 @@ def create_indexed_kernel(assignments,
         return ast
     elif target == 'llvm':
         raise NotImplementedError("Indexed kernels are not yet supported in LLVM backend")
-    elif target == 'gpu':
+    elif target == 'gpu' or target == 'opencl':
         from pystencils.gpucuda import created_indexed_cuda_kernel
         idx_creator = indexing_creator_from_params(gpu_indexing, gpu_indexing_params)
         ast = created_indexed_cuda_kernel(assignments,
@@ -202,6 +204,12 @@ def create_indexed_kernel(assignments,
                                           coordinate_names=coordinate_names,
                                           indexing_creator=idx_creator,
                                           use_textures_for_interpolation=use_textures_for_interpolation)
+        if target == 'opencl':
+            from pystencils.opencl.opencljit import make_python_function
+            ast._backend = 'opencl'
+            ast.compile = functools.partial(make_python_function, ast, opencl_queue, opencl_ctx)
+            ast._target = 'opencl'
+            ast._backend = 'opencl'
         return ast
     else:
         raise ValueError("Unknown target %s. Has to be either 'cpu' or 'gpu'" % (target,))
@@ -298,7 +306,7 @@ def create_staggered_kernel(assignments, target='cpu', gpu_exclusive_conditions=
         outer_assignment = None
         conditions = {direction: condition(direction) for direction in stencil}
         for num_conditions in range(len(stencil)):
-            for combination in combinations(conditions.values(), num_conditions):
+            for combination in itertools.combinations(conditions.values(), num_conditions):
                 for assignment in assignments:
                     direction = stencil[assignment.lhs.index[0]]
                     if conditions[direction] in combination: