[OpenCL] Moves all OpenCL kernels to *.cl files.

Kernel files are located in opm/simulators/linalg/bda/opencl/kernels.
CMake will combine them for usage in
${PROJECT_BINARY_DIR}/clSources.cpp that becomes part of the library.

opm/simulators/linalg/bda/opencl/kernels/axpy.cl

@@ -0,0 +1,15 @@
+/// axpy kernel: a = a + alpha * b
+__kernel void axpy(
+    __global double *in,
+    const double a,
+    __global double *out,
+    const int N)
+{
+    unsigned int NUM_THREADS = get_global_size(0);
+    int idx = get_global_id(0);
+
+    while(idx < N){
+        out[idx] += a * in[idx];
+        idx += NUM_THREADS;
+    }
+}