Implement Pinned GPU memory

CPU arrys with an equivalent GPU array should be pinned. Further, this MR fixes non-aligned strides between CPU and GPU arrays.

pystencils_tests/test_datahandling.py

@@ -387,3 +387,18 @@ def test_array_handler():
 
     random_array = array_handler.randn(size)
 
+    cpu_array = np.empty((20, 40), dtype=np.float64)
+    gpu_array = array_handler.to_gpu(cpu_array)
+
+    assert cpu_array.base is None
+    assert gpu_array.base is None
+    assert gpu_array.strides == cpu_array.strides
+
+    cpu_array2 = np.empty((20, 40), dtype=np.float64)
+    cpu_array2 = cpu_array2.swapaxes(0, 1)
+    gpu_array2 = array_handler.to_gpu(cpu_array2)
+
+    assert cpu_array2.base is not None
+    assert gpu_array2.base is not None
+    assert gpu_array2.strides == cpu_array2.strides
+