Improve the yumi_cameras FTP server code

[YoshuaNava]

When I worked on this package, it was required to have the cameras running as quickly as possible. Hence, at this moment the FTP server code is pretty straightforward, and does what's expected, but does not follow best practices (for example). It could become a class, with comments, and enough functions to start, stop and restart the server.

The C++ code for triggering the cameras could also be encapsulated and documented better.