Expanded Functionality for connection_category

[protocol_1903]

connection_category is a great tool for fluid manipulation and I love it. It has simplified my mod No Pipe Touching down to a single data stage logic loop. Thanks, I couldn't have asked for a better implementation. However, it does have a few seemingly arbitrary limits.

Firstly, underground connections can only have one connection_category. This seems rather arbitrary and I haven't seen a good reason why, can someone explain that one?

Secondly, I think that a blank connection_category should connect to everything instead of everything having the "default" category. That way, assuming the underground connection category limit is unchanged, you can have so-called "universal" underground connections. This is relevant because I'm working with TheStaplergun for compatibility between NPT and Advanced Fluid Handling, which adds an underground pump and a 4-to-4 pipe to ground. Neither of these work with the connection category logic currently in the engine. If this happens, I also wouldn't need to add each individual pipe category to assemblers and pumps and the like, they would just connect to everything automatically. And if you don't want something to connect to these universal connections, perhaps a secondary "connect_to_universal" field should be present in each pipe_connection.