Support classification in .ply files

These MR ensures the classification information that comes from .ply files is considered. It follows !156 (merged), in which the point has been left as a TODO.

⚠ This is of user responsibility to store it into a dedicated classification entry. If we decide to support a generic entry name, this will be the scope for a further MR.

The MR provides a small utility function for building a valid .ply file (i.e. with a "classification" feature).

Related issue: #136 (closed)

Note: that's quite unclear on how to properly test the reader function, because of the entanglement with zmq processes (see #125 (closed)). I've added some additional integration tests on the convert process, as suggested.