Add ability to set multicast source address

[davidjwbbc]

This is a new optional parameter when constructing a Transmitter object to maintain backward compatibility.

When not provided the operation remains the same as before, i.e. pick the address of the appropriate local interface.

When source address is provided:

• When used with a tunnel endpoint this will set the encapsulated source IP address to be used.
• When used without a tunnel endpoint this will bind the output socket to the provided address.
  • An exception is raised if the output socket cannot be bound to the given source address using bind().

Closes #58
Unblocks 5G-MAG/rt-mbs-transport-function#56

[jordijoangimenez]

This is good, @davidjwbbc.
Unrelated to your change but I find now a bit confusing that the multicast group address is just called "address" in the constructor.

Would it be very painful in terms of rewriting to call it eg m_address where used? If painful. Let's go and merge this one.

[davidjwbbc]

This is good, @davidjwbbc. Unrelated to your change but I find now a bit confusing that the multicast group address is just called "address" in the constructor.

Would it be very painful in terms of rewriting to call it eg m_address where used? If painful. Let's go and merge this one.

I'd prefer a different name, maybe something like multicast_address, group_address or destination_address as we have often, in other projects, used the m_ variable prefix to indicate the use of an object member variable and it might get confusing.

Plus it always helps to be a bit more explicit with variable names, rather than trying to abbreviate.

[jordijoangimenez]

Thanks, David. Any of the three options look good. Perhaps destination address? (then we have source and destination more explicit=

[davidjwbbc]

@jordijoangimenez, I've done the destination_address change and updated the code documentation to match. I've also updated the Doxyfile too since my doxygen command was complaining it was old.

[jordijoangimenez]

@davidjwbbc please merge