[2.1] Allow circuit connections to underground belts, consistent with pipes to ground

[ujeenator]

2.1 added circuit network support to pipes, including pipes to ground. Regular transport belts can also be connected to the circuit network, but underground belts still cannot.

This now feels inconsistent: both regular pipes and their underground counterpart support circuit connections, while for belts only the regular entity does.

Underground belts should support the same relevant circuit functionality as regular belts, particularly reading contents and enable/disable.

This would also be useful for compact builds on space platforms and Aquilo, where underground belts are commonly used because of limited space and heating constraints.

[SuperBananaBomb]

+1, for the same reasons and especially compact/aquilo builds.

[Almalexia]

+1