Rename namespace and some vector traits

[AnastasiaStulova]

To avoid future collisions with names in C++ standards and to disambiguate/align better with C++ naming scheme the following rename changes are suggested:

• change (or add nested namespace) namespace opencl for OpenCL specific type traits.
• rename some OpenCL vector traits as follows: -- vector_n -> make_opencl_vector -- is_vector_type -> is_opencl_vector_type -- vector_size -> opencl_vector_size

Alternatively, we could use simd_vector instead of opencl_vector.

[AnastasiaStulova]

if we are to use a separate nested namespace then we might not need to add opencl in trait names?