Fix getConnectee throwing if called during finalizeConnections

[adamkewley]

Fixes an issue I'm having with SocketDefinedFrames implementation (upcoming PR).

Effectively, the bug is that:

• After adding something to a model programmatically, you must call finalizeConnections in order to ensure that the socket pointers are baked into the socket's componentPath property
• Several use-cases in opensim-creator (e.g. undo/redo) call for copying the model, which wipes those pointers
• This is usually fine, because I typically re-call finalizeConnections on those copies
• However, finalizeConnections has very specific semantics that don't work once you have components in the model graph (e.g. custom frames) that depend on components that aren't in the model graph (e.g. Stations):
  • It finalizes the Model's socket connections (non-recursive)
  • Then it creates+traverses a multi-body graph
  • For each frame in the graph, it calls frame.finalizeConnections(model)
  • Which finalizes the frame's sockets, and the frame's children's sockets
  • However, it doesn't finalize the sockets of anything the frame is connected to (i.e. the socket finalization performed during multibody graph building only satisfies the data hierarchy, not the socket graph)

So, in the edge-case that you:

• Have a custom component that will participate in the model graph (e.g. StationDefinedFrame)
• And that component depends on stuff via SOCKETs (e.g. Stations somewhere else in the hierarchy)
• And calling finalizeConnections on the component requires pulling information from those dependencies via those sockets (e.g. calling finalizeConnections on a StationDefinedFrame may want to query the Stations for body-fixed positions, base frame, etc.)
• Then it'll fail

The solution is PRed, and it effectively relaxes sockets such that they work fine even if you don't call finalizeConnection on them. This effectively downgrades finalizeConnection from "hard requirement for using a socket" to "performance-enhancing requirement".

Brief summary of changes

• Make Socket::getConnectee work, even if finalizeConnection hasn't been called on it yet

Testing I've completed

• The OpenSim test suite
• Which had a failure because one test expects sockets to fail if used before finalizeConnection. This test was commented out because that isn't a strictly required behavior (it's a desirable one, for perf reasons, but it isn't a hard requirement)

Looking for feedback on...

CHANGELOG.md (choose one)

• updated.

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This change is 

[adamkewley]

Need to check what's the deal with MacOS/Ubuntu

[adamkewley]

@nickbianco this is ready for review if you're willing!

The function was moved to Component.h, because I noticed that other component-dependent templated functions for Socket<T> appear there. A test (for the failing behavior) was also commented out (because it's not longer a strictly failing case, but a slow one).

Hopefully, this relaxation of the connection finalization behavior is acceptable? I think there's some (reasonable) debate over whether it should be allowed (because it may encourage people to skip finalizing connections), but the counterpoint is that any code that (e.g.) calls buildSystem will be finalized anyway.