Handle namespaces better

[diivm]

`pcl` or any namespace inside which we are?

Does pybind complain if `pcl::` is present? What about in case of `pcl::detail`?

Originally posted by @kunaltyagi in https://github.com/divmadan/pcl/pull/8

Also,

• change handle_namespace functionality
• use of namespaces across bindings (use of flags, etc.)