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_tests/test_cudagpu.py

 import numpy as np
+import pycuda.autoinit
 import pycuda.gpuarray as gpuarray
 import sympy as sp
 from scipy.ndimage import convolve