Boiler prototype, output_fluid_box must be optional

[darkfrei]

From this discussion: https://mods.factorio.com/mod/SteamRehe ... 48073f2ff9

The prototype has value mode = "heat-water-inside", but the output output_fluid_box must be defined and will be ignored.

You can put the steam into the A and got the hotter steam from B, or put into the B and get hotter steam from the A, but:
1. you cannot get steam from output C output_fluid_box;
2. you cannot delete output C output_fluid_box.

Also the target_temperature = 165 will be ignored, but must be defined.

[boskid]

You can have dummy output_fluid_box: one that is defined but has no pipe_connections. Target temperature could be used in the "heat-water-inside" mode because right now boiler will heat fluid up to its prototype max temperature. This mode is not heavily tested and has quite low priority right now.