FlatTrace: possible points of confusion when editing existing object

For example, shifting edits the internal trace, but not `trace_pos`, but changes to `trace_pos` and `trace` do not affect each other:

```
from specreduce.utils.synth_data import make_2dspec_image
from specreduce.tracing import FlatTrace

img = make_2dspec_image()

trace = FlatTrace(img, 10)
print(trace.trace_pos, trace.trace[0])
# 10, 10

trace_shifted = trace + 10
print(trace_shifted.trace_pos, trace_shifted.trace[0])
# 10, 20

trace_shifted.trace_pos = 15
print(trace_shifted.trace_pos, trace_shifted.trace[0])
# 10, 20
```

Updating the trace_pos during a shift (or addition) can be done easily by overloading `Trace.shift`. Do we also want to implement a setter on the `trace_pos` attribute to update `trace.trace`? What about vice-versa? Or do we make `trace.trace` read-only and require any edits to be by setting `trace_pos` or shifting?

Start by reading FlatTrace and the behavior of Trace.shift shown in the example. Resolve a consistent editing contract for trace_pos and trace, including whether trace is writable, then verify the chosen behavior against the example.

Written by the indexing model from the issue text.