Fix RepeatedKernelRetrieval error in OpenCL

[pkienzle]

Cache the kernels as well as the program so that we avoid the following RepeatedKernelRetrieval warning:

sasmodels/model_test.py::ellipsoid[opencl]
  /.../sasmodels/kernelcl.py:469: RepeatedKernelRetrieval: Kernel 'ellipsoid_Iqxy' has been retrieved more than once. Each retrieval creates a new, independent kernel, at possibly considerable expense. To avoid the expense, reuse the retrieved kernel instance. To avoid this warning, use cl.Kernel(prg, name).
    functions = [getattr(program, k) for k in names]

Note: this code is used by SasView