Clarification of O2device class about aliceo2

[mkrzewic]

Hi,
The purpose is:

• currently: to simplify the API. The AddMessage() method is not static, it uses FairMQ functionality, a standalone version would have to pass an additional parameter (something like "this" or a pointer/ref to a transport factory).
  The ForEach template is kinda standalone, agreed. Also there is no reason for the callback to be a member function, but this is easily changed.

• in general: this would be a place to implement any other (subtle) differences to FairMQ which might (and most probably will) pop up in the future in a way completely transparent to the (O2-) devices.

That being said, I was also initially opposed to having an O2Device class until I started playing with the code - this seems a tad more elegant and simple.

[sawenzel]

@mkrzewic:

a) Thanks. Indeed the NewMessage member function is called so I understand the reasoning.

b) The class name should still be fixed.

[mkrzewic]

done, #234