Fix setting socket values

[ElliottKasoar]

Resolves #131

As discussed in the above issue, when tasks are run, their links are evaluated, which take precedence over input values directly set, leading to confusing behaviour where you can set and view values that are disregarded.

At the point where these values are set in _set_socket_value, it doesn't seem to be entirely straightforward to determine whether the update is being triggered by an update to the linked value, or if a value that should be linked is being directly set (and so ignored).

My solution has therefore been to check, for inputs, whether the input being changed corresponds to one in the socket links.

As discussed with @superstar54, the test_operation and test_operation_comparison were not testing quite what we expected, since task.execute does not resolve links, which meant that removing result._task.set_inputs({"x": 4, "y": 2}) raises a key error.

I've updated these to run using a local engine, giving the expected behaviour, and added new tests that check that we can set unlinked values, but not linked values.

[superstar54]

Thanks for the fix! I add two minor suggestions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.54%. Comparing base (d256df2) to head (b9fdda7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #132      +/-   ##
==========================================
+ Coverage   90.46%   90.54%   +0.07%     
==========================================
  Files          73       73              
  Lines        7088     7125      +37     
==========================================
+ Hits         6412     6451      +39     
+ Misses        676      674       -2     

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[ElliottKasoar]

I think we still need the inputs check, otherwise we get errors e.g.

>       result = engine.run(ng=test_op.build())

tests/test_socket.py:509: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
src/node_graph/engine/local.py:76: in run
    return self._finalize_graph_success(ng, graph_pid, graph_outputs)
src/node_graph/engine/base.py:102: in _finalize_graph_success
    ng.outputs._set_socket_value(cleaned)
src/node_graph/socket.py:1271: in _set_socket_value
    target._set_socket_value(val)
...
...

E               ValueError: Input graph_outputs.outputs.result_1 has already been set via a link. Please update the linked value in [TaskLink(from="op_ge.result", to="graph_outputs.result_1")]

[superstar54]

Nice!